example shaders not compatible with WebGL2

[aardgoose]

Out of curiosity I have tried running the examples using a WebGL2 context, several fail. The most common problem is the use of ShaderMaterials with a uniform 'texture' which conflicts with the GLSL ES 3.0 texture() function (easily fixed by renaming the uniform of course).

I think it would be good if examples avoided this issue, given the confusion that may occur if/when examples are used as the basis for other's code.

Thoughts?

[yomboprime]

GPUComputationRenderer.js is guilty here.

[mrdoob]

Yeah, we should definitely clean that up.

[aardgoose]

I'll sort them out shortly