Add 16, 24 bpp support

[paulstraw]

Is your feature request related to a problem? Please describe.
16 and 24 bpp files aren't currently supported. Realistically, 8 bpp max makes the most sense on Playdate, but it'd be nice to have more broad support.

Describe the solution you'd like
Add support for files with those bit depths.

Describe alternatives you've considered
The alternative is not implementing them, which I've already done. :)

Additional context
Nothing too crazy here, but it's a bit of a pain because of configurable color masks:

• 16bpp defaults to a 5-5-5 encoding, but can be specified through the "palette" color masks (e.g. 5-6-5 to use all 16 bits, or something else weird)
• 24bpp is similar
• 32bpp is currently implemented, but I think it would error if given a mask where any of the colors were not 8 bits. I don't know whether anyone has ever actually made a BMP like this, but it might be worth just fixing anyway.