Fix a crash and a limitation in RGBGFX unoptimized output

- If both banks 0 and 1 were completely filled, then
  `assume(bank == 0)` would false and potentially crash.
- If the input image had fully-background tiles, they would be
  incorrectly counted towards `nbTiles` and could cause the
  "Image contains N tiles, exceeding the limit" check to fail.

test/gfx/complete_unoptimized.flags

@@ -0,0 +1,3 @@
+-B transparent
+-c dmg
+-N 256,256