Implement GLES3 functions:

* glGetActiveUniformBlockName
* glGetActiveUniformBlockiv
* glGetIntegeri_v
* glGetUniformBlockIndex
* glGetUniformIndices
* glUniformBlockBinding
Here `uniformBlockName` might be a null pointer, which is ok after this pull request lands https://github.com/kripken/emscripten/pull/2948