This pull request replaces the old `previous_frame` and `current_frame`
with a new pair of `rendered_frame` and `next_frame` that are swapped
once we are ready to draw a finished frame.

This makes it clearer when to use which: `rendered_frame` should be used
to query the existing state, whereas `next_frame` should be used and
mutated when redrawing a dirty window.

The change was prompted by a bug I encountered in
`FocusHandle::contains`. The implementation was reading `current_frame`,
but that was the wrong field to access if e.g. we were reading it inside
of a `render` function or any other time while drawing the window.

Release Notes:

- N/A