Custom background image for options menu

[nicholatian]

Objective: Enable the placement of any 4-bit 240×160 image in the main menu, and ensure it shows up behind the already present rbox construct

Bonus: Enable the placement of any 8-bit 240×160 image, as described above.

Notes:

• Since the rbox data is uploaded to VRAM:06004000 by default, rearrangement chores will need to be done to fit the required number of tiles (601) into VRAM.
• If the rbox tiles are moved to VRAM:06008000, there will be a 512-tile limit on 8-bit images.
• It may be possible to rearrange the VRAM to have the main menu rbox/text tileset at VRAM:0600C000, though I don’t know if there would still be space for the tilemap after that.