backend/render: Code documentation + minor code changes.
backend/render: Code documentation + minor code changes.

This code isn't used yet, but the idea is to have a common code path
for resolving exactly how text is to be rendered by a frontend.

codex8/lime-backend,ch1bo/lime-backend,codex8/lime-backend,bj7/lime-backend,codex8/lime-backend,ch1bo/lime-backend,ch1bo/lime-backend,bj7/lime-backend,limetext/lime-backend,limetext/lime-backend,codex8/lime-backend,bj7/lime-backend,ch1bo/lime-backend,bj7/lime-backend,limetext/lime-backend,limetext/backend,limetext/lime-backend