stop vclcanvas from ruining alpha of bitmaps in Skia mode
According to the comment and tdf#45219 vclcanvas deliberately forces
alpha to be 1bpp because of poor performance. But that's not the case
with Skia, alpha is always 8bpp there, so all the hack does it make
animations look ugly and it also breaks caching capabilities of the
Skia backend because the hack always generates a new bitmap.
I don't know which VCL backend the original problem was related to,
so at least disable the hack for Skia.
Change-Id: I8a90c32dbe911cb79b617c0c447f3e9ced05f746
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/125373
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>