Allow RGBGFX to output SNES attribute maps

Tilemaps can store 8-bit tile IDs. Attribute maps store any high `T`ile bits, plus `P`alette ID (0-7) and `X` and `Y` flip. (There are other bits, like priority, but they're not relevant to RGBGFX.)

Attribute map byte formats:
- GBC: `%.YX.Tppp` (with `T` being a VRAM bank 0/1 flag, not really a high tile bit)
- SNES: `%YX.pppTT`

RGBGFX could have a `--snes` flag to output SNES attribute maps. This flag would imply `-N 1024,0`, and the high two bits of tile IDs would go in the attribute map, not the tile map. (There could be a warning/error if the user explicitly specifies `-N`, or outputs a tilemap but no attrmap.)