GLSL shader generator allows for invalid "__" substring in identifiers

If a shader node has a name with "__" substring within the it's name the GLSL shader generator will generate
identifiers with that string in it. This is not allowed so shader compilation will fail.
Error is:
```
error C7528: OpenGL reserves names containing '__'
```

Start by locating the GLSL shader generator and the code that constructs identifiers from shader node names. Reproduce the issue with a node name containing "__", then verify that generated identifiers no longer contain the reserved substring and that shader compilation succeeds.

Written by the indexing model from the issue text.