Add base value to set alpha channel for 32 bit colourspaces.
Add base value to set alpha channel for 32 bit colourspaces.

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

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core