This is an effort to make SalGraphicsImpl mandatory for all
backends.
This introduces Qt5GraphicsBackend: a subclass of SalGraphicsImpl,
which now handles graphic rendering.
Change-Id: I42aece59d0c692ca1dd33e30f31c5bcceab02008
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/113734
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>