Mojo OpenGL entry point / header file refactors

[James Robinson]

The way we currently expose OpenGL functionality to Mojo applications has some issues.  The mechanism requires application developers to use header files that are not provided in the Mojo SDK and the organization makes it difficult to expand to API layers above ES 2.0 or functionality like OpenGL share groups.  I've spent a bit of time studying this problem and approaches on other platforms and have written up this proposal: https://docs.google.com/document/d/1IqTqm17quekPP3rw2oOaQbDkHVy_I0aBcPp8NlESlCI/edit?usp=sharing.  Executing this proposal requires changing around the public SDK and will thus require some coordination with SDK consumers to make sure they can transition.  If the proposal appears to have consensus, I'll start working with Mojo SDK consumers to adapt to the new organization in a smooth fashion.

- James