Overhaul of texture image handling.

Overhaul of texture image handling.
1. gl_texture_image struct's Data pointer points to images in driver's format.
2. Added FetchTexel() function pointer to struct gl_texture_image.
3. Changed Driver Tex[Sub]Image functions, return void now.
4. Texture storage/fetch code in new texstore.c file.
5. Removed texture.[ch] - functions moved to state.c

Note: FX driver updates not finished yet.