This required a significant refactor to move the mapper data out of the static
struct, but in the end is a saner architecture.

- Gun-Nac working great
 - Kirby menus are garbage
 - SMB3 overworld sprites are garbage

- Gun Nac still crashing after first title screen
 - SMB3 halts after game start

- Scrolling jitter artifacts evident in The Legend of Zelda

- Ready to add support for mappers
 - Block-line background rendering presents certain artifacts in some
   vertical-scrolling games like 1942

- DMA was being double-counted for PPU cycles