Add more RGBGFX tests

[Sylvie]

Run contrib/coverage.bash to see exactly what still needs testing.

• Reverse mode
  • 1-bit depth
  • GBC color curve
• Reading palette file formats (note that Tilemap Studio can help in generating these)
  • ACT
  • ACO
  • GBC
  • PSP
• -X and -Y for limited mirroring
• -i/--input-tileset
• -B/--background when that's implemented