svp: deduplicate bitcount->colourspace mapping and allow overriding.

Although svp defaults to BGR, we might want to use alternative
formats (e.g. for tiled rendering to bitmap buffers which are
to be used in e.g. gtk), it is probably safest to keep the current
defaults but allow the user to change to whatever format they may
require. (This currently only makes sense for the 32-bit
RGBA/ARGB/etc. formats. However the 23 bit formats could potentially
be expanded to allow a similar RGB/BGR choice.)

Change-Id: I70bd3d6e7d297faef163b910f576655efee4cb3f
Upgrade LibLO tiled rendering to RGBA.

This is the expected format for gtk+ pixbufs which is currently
the primary target. We could potentially allow the user to choose
the format via LibLO -- but we want to eventually allow passing
in a buffer (rather than passing out an LO owned buffer) anyway, so
should add that then.

Change-Id: I8427925a94366917fa82fb8ea28e7dbb3fa1840d
Upgrade gtktiledviewer to RGBA.

The Alpha channel seems to be set incorrectly by LO, hence
we need to manually set it here for now.

Change-Id: I1f9091b8b6f88c1dba6653dfb7bf51f9fe14b3fc