Ignore ‘game’ variable when doing ‘resetall’.

Changing ‘game’ from inside the loop is especially bad since it triggers
config execution.
Clean up and improve image management code.

Move 8-bit image inpacking and skin flood filling code into shared
renderer code. Mark paletted PNG images with IF_PALETTED flag. Remove
dependency on zlib.h. Remove duplicated ‘r_texture_formats’ entries.
Print full path when R_RegisterImage fails with error. Fix negative PCX
width/height. Don't search for alpha when image is known to be opaque.