terminal: clear unused margins on each frame

terminal: clear unused margins on each frame

If the terminal screen is smaller than the real screen, we never paint to
the margins. This doesn't hurt as long as we never resize the terminal.
The uterm layer clears all framebuffers during allocation.

However, uterm behavior may change and our terminal may get resized (eg.,
during hotplugging) so we really should clear all the margins.

We now clear them on every frame as it is a trivial task. However, if we
speed up rendering, we should probably set a "needs_clear" flag that
simply clears the framebuffer.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>