Add setting for how to render fully transparent pixels

[GriffinRichards]

Closes #654

The GBA will render fully transparent pixels using palette 0 color 0, but the Pokémon games overwrite palette 0 color 0 with black. One of the options for the new setting (and the default) will render transparent pixels as black, the other will render them with whatever is in color 0.

Additionally:

• Adds a new setting for how to render the unused layer for metatiles. This was previously hard-coded to 0x3014 for Normal metatiles, and 0x0000 for Split/Covered metatiles.
• Adds some warnings if the fieldmap properties (number of tiles, number of primary palettes, etc.) are given nonsense values
• Disables the tileset divider, until the metatile images are actually separated by tileset.