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