Make the fragment shader in this sample spec compliant

The fragment shader in this sample was calling pow(x, y) where x < 0
but according to the spec pow(x, y) where x < 0 is undefined.

This sample therefore didn't run on iOS where that GPU looks like it
it returns 0 or NaN when x < 0
Merge pull request #5626 from greggman/fix-webgl-shader

Make the fragment shader in this sample spec compliant