Slider/text UC: for integer-only variants, restrict node to integer positions

There aren't many places where this is relevant, but in those few
places, this makes the slider a bit more intuitive.
Effects > Artistic > Figured glass: convert to new supersampling engine

All things considered, it's pretty easy to drop in the new engine.  Yay!
Effects > Transform > Pan and Zoom: integrate supersampling engine

This tool is probably PD's best showcase of the benefits of
supersampling.  Take any large image, use this tool to zoom out to the
max level (-10) and compare the output of Quality 2 (no supersampling)
with Quality 5 (3 grid / 12 pt supersampling).

Quite a difference!