BIOSF_CLEARFIX ($C004C2): Clear fix layer
Warning: register contents aren't preserved ! (Uses D0, D1, and A0)
Clears all the fix layer with tile # $FF, palette 0. The leftmost and rightmost columns are cleared with tile # $20, palette 0.
In the default fix tilesets, tile # $FF is filled with color 0 (transparent). Tile # $20 is opaque (used for masking).