Ternary/graded shadow support

I added support for a third (mid-range) level to shadows, for alpha values in the shadow-caster which are reasonably semi-transparent.

BRANCH IS HERE: https://bitbucket.org/tofu_linden/viewer-development-plus-gradedshadows

This works fine and stuff, and it's of extremely similar cost to the binary shadow (it doesn't incur any more texture reads etc).
The downside is that the pcf sampling position is a bit more quantized along one of the shadow map axes. I added some handwaving to hide that fact (disguising the loss of freedom with some more fuzzing, including turning the pcf function into a plain averaging function and adding some jitter).

I'll do some comparison screenshots and stuff when I have the time/energy.