cursor-renderer: Align OpenGL cursor rect to physical pixel grid

When stage views are scaled with fractional scales, the cursor rectangle
won't be aligned with the physical pixel grid, making it potentially
blurry when positioned in between physical pixels. This can be avoided
by aligning the drawn rectangle to the physical pixel grid of the stage
view the cursor is located on.

Fixes: #413

!610
3 jobs for master in 4 minutes and 24 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #321136
build-mutter

00:01:41

 
  Test
passed #321138
can-build-gnome-shell

00:01:30

passed #321137
test-mutter

00:02:43