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.