Add OpenGL support

Is there any plan to support OpenGL contexts / widgets in `toga`? There is code for creating them for OSX/Linux/Windows in `Pyglet`, so there is some guideline available. Just curious to know if I should keep an eye on it, because if OpenGL contexts are available then it could be made to operate with / as a backend for [vispy](http://github.com/vispy/vispy).

Start by reviewing the existing Pyglet code for creating OpenGL contexts and widgets on OSX, Linux, and Windows, then compare it with toga's backend structure. Clarify the scope of OpenGL support and how it would operate with or as a backend for vispy; done requires an agreed design and implementation criteria.

Written by the indexing model from the issue text.