WebGLRenderer: Fixed shader compile error.

[elalish]

This is a port of this fix: google/model-viewer#920
It was causing the PMREM shaders to fail compilation on an Alcatel A502DL, which I was able to repro on three's gltf loader example. Oddly, it does not fail the MeshStandardMaterial compilation, even though it uses the same shader fragment, so this is a pretty strange driver bug. It doesn't even give an error or line number when the compilation fails, which made finding this fix a bit tricky. Thanks to @cdata for his help hunting it down line by line when he was the only one with a repro device.

[mrdoob]

Thanks!

[mrdoob]

FYI @kenrussell

[kenrussell]

Interesting - would this be reproducible with a small shader snippet using the framework from the WebGL conformance tests here:
https://github.com/KhronosGroup/WebGL/tree/master/sdk/tests/conformance/glsl/bugs

Or does this need more of the shader in order to reproduce?

If you can boil this down to a small failing shader, please file an issue on that repository and/or put up a pull request adding a test covering this bug. Thank you!