This adds an test for a VirtualDevice with an alpha channel,
which checks that getting a BitmapEx from the alpha VirtualDevice
is has a alpha channel properly set.

Test that using GetPixel and DrawPixel properly handle an alpha
based Color, which they didn't, so this also includes the fix for
thouse 2 methods.

Change-Id: I419b8e0f66ab5f8266c6129e501ed75ef517fd44
Reviewed-on: https://gerrit.libreoffice.org/70805
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>