- removed deprecated bitmap arg from IMB_allocImBuf (plugins will need updating).
- mostly tagged UNUSED() since some of these functions look like they may need to have the arguments used later.
Revised external multires file saving. Now it is more manual in that you
have to specify where to save it, like an image file, but still saved at
the same time as the .blend. It would ideally be automatic, but this is
difficult to implement, so for now this should at least be more reliable.
