In the do_clip() function, texelFetch is sometimes returning
invalid results on some nVidia GPUs.

To work around that, use UVs as normalized float coordinates
and fetch via texture(). This fixes the problems on nVidia
cards.

This exposed a bug in how we handle vertex snapping with clips.
Previously, we just cast the float UVs to an int before passing
to texelFetch. Now, pass in the snap offset and apply that to
the UVs for the clip mask.