Relax OpenGL requirements to support more platforms about avara

[assertivist]

@shrizza has noted that the game runs fine in software rendering mode, but the nanogui portion of the app is very slow.

There is a fork of the nanogui library that supports OGL ES, however where we have made changes to nanogui to fit into SDL better, they have made changes to it to fit into GLFW. It may be a painful merge.

[assertivist]

arescentral/antares@b25d16c#diff-62173b77eb9c786aad6e9a07f41b1f3a

Antares, a fellow open source modernization effort, recently made changes to support OGL and compile to ARM platforms that will probably be useful for us to look at!

[assertivist]

I'm now looking into a library that is like ANGLE but supports more backends (and also nanovg as a frontend)
https://github.com/bkaradzic/bgfx

[assertivist]

An update on this issue, as part of the effort to deprecate nanogui, I have simply pulled in the OpenGL specific support we were using from that library into our own application. As a result, I was also able to downgrade the OpenGL version required from 3.3 to 2.1, which allows the application to run on a considerably larger/older pool of devices. So this will be done when the new user interface is complete.