clean up some DDS error messages and other minor bug/comment fixes
move bitmap stuff from pstypes.h, it makes more sense here anyway
start of 8-bit palette conversion, needs work but is here for WMCoolmon to look at and play with
- use VM_* macros for bmpman since it didn't seem to register the memory correctly (temporary)
- a little "stupid" fix for dds bitmap reading
- fix it so that memory is released properly on bitmap read errors
- some cleanup to model texture loading
- allow model textures to get released rather than just unloaded, saves bitmap slots
- bump MAX_BITMAPS to 4750, should be able to decrease after public testing of new code