Allow embedded clients to specify which windows should be redrawn.
Added some new API to nux to allow embedded clients (eg, compiz) to specify which BaseWindows should be redrawn on a frame. This is done by passing a list of rectangles to nux which represent the redraw region, and nux will mark all intersecting windows as needing re-presenting to the screen. This means that we aren't re-presenting every window to the screen on every frame (bad for performance, and also doesn't work correctly in the buffer-swap case).
Also added API to allow nux to tell the difference between draw and read framebuffers when it plays around with the framebuffer object binding. This is necessary in case the embedded client expects the read framebuffer binding to remain defined even after nux has changed the draw framebuffer binding.