Fix graphics format

The fix layer uses 8x8 pixels tiles, coded in a 4BPP linear format with mixed columns. Color indexes are coded by pairs and by columns from top to bottom. Note that the pixel positions are swapped in each byte (L/U) and that the first column is column A.

Size

One tile is 8 * 8 pixels * 0.5 byte per pixel = 32 bytes.

Addressing

Bit # ...543210
      ...nHCLLL

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

2H1 is "C".