BG: support more BMP files

[OliveIsAWord]

Currently, bg.fxf only supports BMP files with a bit depth of 24 or 32 bits per pixel, and also does not support any of the "compression" types. It also incorrectly handles images of negative height (meaning the image data is stored top to bottom). At least some of these, especially 2/4/8 bits per pixel images, should be supported.

[OliveIsAWord]

We also don't properly handle images whose width is not divisible by 4! This should surely be fixed.