The size and position of the window rectangle for clipping in
try_pushing_rect_as_window_rect is calculated by projecting the
rectangle coordinates. Due to rounding errors, this can end up with
slightly off numbers like 34.999999. These were then being cast
directly to an integer so it could end up off by one.

This uses a new macro called COGL_UTIL_NEARBYINT which is a
replacement for the C99 nearbyint function.