Fix glsl. # != //.
Support up to 4 datasets.
Refactor operation order.
Support for value 0.0.
Add benchmark.

Whoa, there's massive overhead with this. @hughsk you did warn me. Will
need to come up with more compelling cases where glsl is actually
faster.
Reduce precision check in benchmark to 1/2 float.
Release 1.0.0-alpha2