Currently the alpha channel is completely ignored by basebmp.
However this results in completely "transparent" output, meaning
the client has to manually overwrite the alpha channel -- instead
we now set it automatically when writing colourdata.
Unfortunately this doesn't quite work -- it seems that drawing
a non-opaque bitmap/image on top of the existing bitmap can
erase the alpha channel information (i.e. these areas will
once again be transparent -- for example document borders seem
to have a transition effect overlayed onto them): presumably
there is some method that bypasses our RGBMaskSetter (probably
some form of direct manipulation of raw values?).
manipulation in basebmp
Change-Id: Ia4be6a748cc30191a4422121f9ec347d9198b225
Equality test also needs to check if disjunct BitmapDevice instances
might not actually share the same memory buffer.
Change-Id: I09a93cb092a0039353be211ed053e991e7fe66f0
see https://gerrit.libreoffice.org/#/c/3367/
and Change-Id: I00c96fa77d04b33a6f8c8cd3490dfcd9bdc9e84a for details
Change-Id: I199a75bc4042af20817265d5ef85b1134a96ff5a
