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

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

- Still some lingering issues with vertical page mapping

- a -> A
 - s -> B
 - q -> Select
 - w -> Start
 - Arrow keys

- Master clock cycles are truncated to nanoseconds, but this should
   be within tolerance of actual hardware implementations