This commit 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.