Fix graphics format

The fix layer uses 8x8 pixels tiles. They're coded in a 4 bits per pixel, linear format with oddly mixed columns. Pixels are coded by pairs as bytes, in columns from top to bottom.

Note that the pixel positions are swapped in each byte (the left pixel goes in bits 0~3, the right pixel goes in bits 4~7), and that the first stored column is column A.

Size

One tile is 8 * 8 pixels * 4 bits per pixel = 256 bits = 32 bytes.

ROM addressing

Bit # ...543210
      ...nHCLLL

n: Tile number (multiple bits)
H: Half, 0 = right, 1 = left
C: Column in half, 0 = left, 1 = right
L: Line number (0~7)

2H1 is "C".