Fix for image replacement in sprites, now the sprite extension is stripped before...

Fix for image replacement in sprites, now the sprite extension is stripped before the frame names are generated (so naming for s_explod.spr is like s_explod_0.tga, s_explod_1.tga, etc) and the extension stripping in the image loader will only strip off .lmp or .pcx or .tga, not other extensions, the sprite extension stripper is similar but for .spr and .spr32.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@127 d7cf8633-e32d-0410-b094-e92efae38249