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Add test for creating gl contexts in a main loop and using -s FULL_ES2=1. This will fail with the following JavaScript exception "Uncaught TypeError: Cannot set property '0' of undefined" in GL.newRenderingFrameStarted()
Test for a current GL context before beginning a new rendering frame.
Merge pull request #2810 from ricochet/no-current-context

Handle no current context when rendering in a main_loop