New glTexImage code.

New glTexImage code.
The gl_texture_format struct now has a StoreTexImageFunc that's called
by glTex[Sub]Image[123]D to convert the user's texture data into the
specific texture format layout.  Now it's much easier to add new texture
formats (like the 16/32-bit floating point formats).
The texutil.[ch] and texutil_tmp.h files are obsolete.