This is intended to replace the ImplPixelFormat in macOS and
OpenGL backends, which are duplicated. It'll also come in
handy for other situations in the future.

Also ImplPixelFormat4 (4-bit Palette) in the macOS backend
doesn't work and never has worked correctly (bug at writing).
This fact was discovered while testing the ScanlineTranformer,
which was based on the ImplPixelFormat* code.

Change-Id: I96a172bd61c2cc116ab3e8b9f47f23f945243e96
Reviewed-on: https://gerrit.libreoffice.org/71992
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>