Deal with various .bmp file format variants in SDL_LoadBMP_RW(). This helps when modern versions of The Gimp (and lots of other things) produces a 32-bit bitmap with an alpha channel, or anything with "BI_BITFIELDS" format, since that data is now embedded in the bitmap info header instead of directly following it and we would accidentally skip over embedded versions of it. Fixes Bugzilla #2714.