Tilegen shaders are mapped to uniforms, and the vram and palette are mapped to two textures.
TODO rip out the redundant code in the tilegen class. We don't need to pre-calculate palettes anymore. etc
The tilegen code supports has a start/end line so we can emulate as many lines as we want in a chunk, which will come in later as some games update the tilegen immediately after the ping_pong bit has flipped ~ 66% of the frame.
The scud rolling start tilegen bug is probably actually a bug in the original h/w implementation, that ends up looking correct on original h/w but not for us. Need hardware testing to confirm what it's actually doing.
