You get BitmapColor only from reading pixels from a Bitmap and
we want to avoid usage of Bitmap outside of VCL (and use BitmapEx
as the alternative which will eventually replace Bitmap).

Change-Id: Iddfa3ef739bfdd4dce5fb47fd9f67a5a36f3388b
Reviewed-on: https://gerrit.libreoffice.org/70447
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>