Get rid of outdated comments.

We can now set the colourspace as desired, and we return the rowstride too.

Change-Id: Idf1e55a67b9e9ab58e82d7ed0be2813b682ec2ff
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