ClientWindow: Compensate UI aspect ratio for side-by-side rendering

The root widget's logical size does not have to be the same as the
window size. Now for side-by-side rendering, the logical height is
doubled.

Also, coordinates in mouse events are now mapped appropriately for
the left/right frames.

Todo: It appears some widgets' clipping and particularly LegacyWidget
do not respect the root widget's size; at least the latter defines
its own projection matrices.