Branch instructions may jump incorrectly and take the wrong cycles, this is because cross function always takes offset paramenter as u8, it should be s8 in branch instructions.
PC is incremented twice before being pushed by BRK, fixes #38.
Sprite overflow flag should be set when there are more than 8 sprites, instead of just 8, see nesdev wiki.
crossfunction always takes offset paramenter asu8, it should bes8in branch instructions.