Lots of changes to use SDL_RWops to load resources, to assist portability later.
Specifically, the PAK files are now read through SDL_RWops.
This means I had to add a bm_load_rw(SDL_RWops *rw) to the bitmap module and add code to load PNG and JPEG files through the SDL_RWops as well.
The bitmap API now has functions for bilinear and bicubic resampling of images.
They're actually intended for another project, but Rengine has the canonical versions of bmp.c and bmp.h for now.
moved bace.c and pakr.c to a separate utils/ directory so that the game's source files are isolated.