Allow RGBGFX to output SNES attribute maps

[Rangi42]

Tilemaps can store 8-bit tile IDs. Attribute maps store any high Tile bits, plus Palette ID (0-7) and X and Y flip. (There are other bits, liek 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.)

[Rangi42]

This would go well with -d 4 support (#1095).

[Rangi42]

If we're keeping a GBC focus, this could be --sgb not --snes, implying -N 256,0 -d 4 (plus max 3 palettes with IDs 4-7).