Round window and geometry positions in OpenGL scene

This rounds the position of geometry in the OpenGL scene, which means
things that are fractionally scaled and end up between pixel boundaries
are instead aligned to pixels. This doesn't fully solve the fractional
scaling blurriness, as we still need the application to provide a buffer
with the correct size. If we do have a buffer with the correct size, we
will now render properly aligned. For example, internal clients should
now be correct.