And make sipify handle this nicely. This means that all our non-flag
style enums correctly map across to IntFlag python enums on Qt 6,
fixing issues with negative enum values for these and providing
a better match for the original c++ enum.
Populating the style library from the database can be a time
consuming process, especially if a user has many symbols
present in their library.
But for many standalone scripts, and for qgis_process, the
style database may not be required.
Let's defer initialization of it until it's actually required,
saving the startup cost in qgis_process and 3rd party scripts.
On my system with a style database containing ~700 items this
cuts down qgis_process startup times by around 25%
