Since f1b59876344bf0c0f066c81bf8ef73ef25365023, we honour DCP HTTP endpoints
exposed in GetCapabilities.
But some (all?) servers fail to add in those endpoints the custom query parameters
the user might have injected in the GetCapabilities URL.
So re-add them manually.
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==
```
In QGIS 2 it was possible to offset features from a non-active layer
which would result in an offset copy of these features being inserted
into the (editable) active layer
This behavior was lost in QGIS 3, which only permits offsetting
features within the active layer.
Resurrect this behavior, with some tweaks:
- the copy-and-offset mode is activated only when ctrl is held
while offsetting. In earlier 3.x versions this also resulted in
copy-and-offset, but only for features within the current layer.
Now, copy-and-offset respects the project's snapping configuration
and potentially copies from other layers
- We use refined logic to correctly map attributes across from
the source feature layer to the destination layer, and apply
default values and provider constraints on feature copy (also
applies to copy-and-offsets where the source and destination
layer are the same)
to create new features
This ensures the newly created features are compatible with the destination layer,
e.g. converting single parts to multipart where required, and also ensures
that default values and provider-side constraints are correctly handled.
