Comment 14 for bug 1731788

I suppose the error about non-constant index is due to the use of `gl_FragCoord.xyy` in the expression for `index`. See https://stackoverflow.com/a/19529963/673852 for description of what may be used as an index. `gl_FragCoord` is neither a constant-expression nor a loop index, so an expression containing it can't be a constant-index-expression.