These methods return a file extension to use when creating vector/raster outputs (e.g. "tif"). Generally,
it is preferable to use the extension associated with a particular parameter, which can be retrieved through
QgsProcessingDestinationParameter::defaultFileExtension(). However, in some cases, a specific parameter
may not be available to call this method on (e.g. for an algorithm which has only an output folder parameter
and which creates multiple output layers in that folder). In this case, the format returned by this
function should be used when creating these outputs.
It is the algorithm's responsibility to check whether the returned format is acceptable for the algorithm,
and to provide an appropriate fallback when the returned format is not usable.
output format
This setting is still used and required in some circumstances, e.g.
it is used as the default selection in outputs file pickers when
the previously used format is not valid.
This should apply to both PROJ < 6 and >=6 builds.
Fixes the following error reported by Valgrind
```
==5848== Invalid read of size 4
==5848== at 0xE5E4195: internal_pj_ctx_get_errno (in /home/even/proj/install-proj-master/lib/libproj.so.15.2.0)
==5848== by 0xE5E16AE: internal_pj_free (in /home/even/proj/install-proj-master/lib/libproj.so.15.2.0)
==5848== by 0xE5F4E78: internal_proj_destroy (in /home/even/proj/install-proj-master/lib/libproj.so.15.2.0)
==5848== by 0x7F961A7: QgsProjUtils::ProjPJDeleter::operator()(PJconsts*) (qgsprojutils.cpp:76)
==5848== by 0x7D2D44C: std::unique_ptr<PJconsts, QgsProjUtils::ProjPJDeleter>::~unique_ptr() (unique_ptr.h:239)
==5848== by 0x7D2C9BD: QgsCoordinateReferenceSystemPrivate::~QgsCoordinateReferenceSystemPrivate() (qgscoordinatereferencesystem_p.h:94)
==5848== by 0x7D2D6FA: QExplicitlySharedDataPointer<QgsCoordinateReferenceSystemPrivate>::~QExplicitlySharedDataPointer() (qshareddata.h:165)
==5848== by 0x7CDFA77: QgsCoordinateReferenceSystem::~QgsCoordinateReferenceSystem() (qgscoordinatereferencesystem.cpp:233)
==5848== by 0x55CC0AF: QgsEllipsoidUtils::EllipsoidParameters::~EllipsoidParameters() (qgsellipsoidutils.h:39)
==5848== by 0x7D8C40B: QHashNode<QString, QgsEllipsoidUtils::EllipsoidParameters>::~QHashNode() (qhash.h:149)
==5848== by 0x7D8C43F: QHash<QString, QgsEllipsoidUtils::EllipsoidParameters>::deleteNode2(QHashData::Node*) (qhash.h:536)
==5848== by 0x9BE3B78: QHashData::free_helper(void (*)(QHashData::Node*)) (in /opt/qt59/lib/libQt5Core.so.5.9.1)
==5848== by 0x7D8C466: QHash<QString, QgsEllipsoidUtils::EllipsoidParameters>::freeData(QHashData*) (qhash.h:576)
==5848== by 0x7D8CCED: QHash<QString, QgsEllipsoidUtils::EllipsoidParameters>::~QHash() (qhash.h:254)
==5848== by 0xA60E369: __cxa_finalize (cxa_finalize.c:56)
==5848== by 0x77BE742: ??? (in /home/even/qgis/QGIS/build/output/lib/libqgis_core.so.3.9.0)
==5848== by 0x4010DF6: _dl_fini (dl-fini.c:235)
==5848== by 0xA60DFF7: __run_exit_handlers (exit.c:82)
==5848== by 0xA60E044: exit (exit.c:104)
==5848== by 0xA5F4836: (below main) (libc-start.c:325)
==5848== Address 0x45db80e0 is 0 bytes inside a block of size 136 free'd
==5848== at 0x4C2F440: operator delete(void*) (vg_replace_malloc.c:586)
==5848== by 0xE5F5A3A: internal_proj_context_destroy (in /home/even/proj/install-proj-master/lib/libproj.so.15.2.0)
==5848== by 0x7F9617A: QgsProjContext::~QgsProjContext() (qgsprojutils.cpp:48)
==5848== by 0xA60E5FE: __call_tls_dtors (cxa_thread_atexit_impl.c:155)
==5848== by 0xA60DF26: __run_exit_handlers (exit.c:40)
==5848== by 0xA60E044: exit (exit.c:104)
==5848== by 0xA5F4836: (below main) (libc-start.c:325)
==5848== Block was alloc'd at
==5848== at 0x4C2E709: operator new(unsigned long, std::nothrow_t const&) (vg_replace_malloc.c:387)
==5848== by 0xE5E445E: internal_pj_ctx_alloc (in /home/even/proj/install-proj-master/lib/libproj.so.15.2.0)
==5848== by 0x7F96151: QgsProjContext::QgsProjContext() (qgsprojutils.cpp:39)
==5848== by 0x7F970A9: __tls_init (qgsprojutils.cpp:31)
==5848== by 0x7F970F3: TLS wrapper function for QgsProjContext::sProjContext (in /home/even/qgis/QGIS/build/output/lib/libqgis_core.so.3.9.0)
==5848== by 0x7F96186: QgsProjContext::get() (qgsprojutils.cpp:57)
==5848== by 0x7D86B13: QgsEllipsoidUtils::definitions() (qgsellipsoidutils.cpp:351)
==5848== by 0x7D863FE: QgsEllipsoidUtils::ellipsoidParameters(QString const&)::{lambda()#1}::operator()() const (qgsellipsoidutils.cpp:173)
==5848== by 0x7D87EE1: void std::_Bind_simple<QgsEllipsoidUtils::ellipsoidParameters(QString const&)::{lambda()#1} ()>::_M_invoke<>(std::_Index_tuple<>) (functional:1531)
==5848== by 0x7D87BC5: std::_Bind_simple<QgsEllipsoidUtils::ellipsoidParameters(QString const&)::{lambda()#1} ()>::operator()() (functional:1520)
==5848== by 0x7D879A2: void std::__once_call_impl<std::_Bind_simple<QgsEllipsoidUtils::ellipsoidParameters(QString const&)::{lambda()#1} ()> >() (mutex:706)
==5848== by 0x10A6AA98: __pthread_once_slow (pthread_once.c:116)
==5848== by 0x7D863CE: __gthread_once(int*, void (*)()) (gthr-default.h:699)
==5848== by 0x7D8787F: void std::call_once<QgsEllipsoidUtils::ellipsoidParameters(QString const&)::{lambda()#1}>(std::once_flag&, QgsEllipsoidUtils::ellipsoidParameters(QString const&)::{lambda()#1}&&) (mutex:738)
==5848== by 0x7D86476: QgsEllipsoidUtils::ellipsoidParameters(QString const&) (qgsellipsoidutils.cpp:174)
==5848== by 0x7E8D572: QgsMapSettings::setEllipsoid(QString const&) (qgsmapsettings.cpp:322)
==5848== by 0x6A4E5F1: QgsMapCanvas::QgsMapCanvas(QWidget*) (qgsmapcanvas.cpp:143)
==5848== by 0x5110841: QgisApp::QgisApp(QSplashScreen*, bool, bool, QString const&, QString const&, QWidget*, QFlags<Qt::WindowType>) (qgisapp.cpp:803)
==5848== by 0x417DB9: main (main.cpp:1339)
==5848==
```
Whilst usually this is bad practice (reusing an existing expression context),
and instead you should make a new context for every batch of expression evaluations,
we can't do this in the style model and we end up potentially creating
thousands of new contexts. So instead, we use a single one, which is
safe because the actual expression context used here is a very
basic one...
Allows restoration of a layer's selection following a selection
clear operation. Handy for anyone who has ever painstakingly
built a custom selection only to accidently click and clear
this selection...
