Approach to built-ins

Currently if a builtin isn't hard-coded it could break shaders (for example, `fwidth` not being listed might get mangled to `fwidth_2_1`). As WebGL2 and new extensions come to browsers, we should be wary of new functions/constants.

Some things not listed in the built-ins:
- pragmas like `GL_ES` and `__VERSION__` -- not sure if that matters tho
- gl_MaxFragmentUniformVectors, gl_MaxVaryingVectors, gl_MaxVertexUniformVectors
- I added dFdx / dFdy, but there is also fwidth
- transpose (GLES 3)

Some other things like gl_InstanceID, gl_ClipDistance, outerProduct, shadow2DProj, etc may or may not eventually end up in WebGL (in spec or through extensions).

Maybe there is another way of handling this? Like only mangling function names when necessary?