This commit is very large, representing about a month of work with many
interdependent changes that don't separate cleanly in to atomic commits.
The main change here is unifying the renderer logic to a single generic
renderer, implemented on top of an abstraction layer over OpenGL/Metal.
I'll write a more complete summary of the changes in the description of
the PR.
"Grid" was a really antiquated name when it had both the screen state
AND the renderering functionality tied together. This hasn't been true
for a LONG time and it is long overdue that this is renamed to its
proper name.
This also begins setting up a folder structure to anticipate future
renderers and rendering functionality. I'm not working on any alternate
renderers right now so the interface isn't expected to be good, just
laying out the files in this way.
