Invadrox - some bitmap related changes in preparation of porting it to more screen...

Invadrox - some bitmap related changes in preparation of porting it to more screen resolutions, should not be visible to the user: (1) Use single bitmaps (or strips where possible) instead of one big bitmap each, so instructions and defines how to take the bitmap apart aren't needed. It's more flexible and makes it possible to (2) get some width/height variables directly from the bitmaps. (3) Use background images for static items. Both reduces the number of hardcoded values noticeably. (4) Make the 'fire_sprite' an external bitmap too - with help of Jens -. (5) Rename 'TINY_GRAPHICS' to 'SMALL_GRAPHICS' to have this name available for even smaller images (which is planned).

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@20646 a1c6a512-1295-4272-9138-f99709370657