Always redraw the UI on orientation change

Bug #4994715

In some cases, on some architectures, the hardware renderer could fail
to properly render a window after an orientation change. The fix is
to force the GL driver to drop any existing buffer to make sure the
next buffer used to draw has the correct geometry.

This change also fixes an issue that was preventing partial screen
updates from being properly enabled.

Change-Id: I874a48590845a8624425130af7db8e20022895dd