External draw buffer

[fengb]

Instead of leaving draw buffer(s) in the "core", Fundude should allow for passing in an external buffer. On vblank, the local display will be flushed into the buffer.

Rationale:

1. Shrink the core memory usage
2. The buffer flush can do the decode, which should be faster than JS (and a lot more readable than https://github.com/fengb/fundude/blob/master/web/Display.tsx#L87-L106)