This adds a test for drawing a BitmapEx which has an alpha channel
onto a VirtualDevice. Most important is that alpha blending is
done as we expect it (but it is prone to rounding error).

Change-Id: I41a16ca9da98d7a067ada9aafad8c9fc7c6484d5
Reviewed-on: https://gerrit.libreoffice.org/70804
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>