Relax OpenGL requirements to support more platforms

[assertivist]

Currently the game will not launch without being able to create an OpenGL 3.3 context. This is a pretty high bar, especially for ARM platforms, which mostly target OpenGL ES.

There are different ways to do this, one way I've read that people do it manually is just to try opening different contexts (OpenGL 3.3, then 2.x, then ES) until one works, and then adjust the shaders with text replacement and feature removal if thats necessary, to get them to compile against the target context.

AFAIK we do not use any features specific to OpenGL 3.3 that are not available in 2.x/ES, and it may be possible to adjust the shaders so that they will work against any of those contexts, but I haven't seen an easily digestible example of this yet.

I've also looked at ANGLE, which is a library Google uses to support graphics acceleration on multiple platforms. Lots of large open source projects use this, and I think it would be good to use, seeing as they have committed to delivering Metal support, which we may need for macos in the somewhat near future. I am not totally sure how much restructure of our code would be necessary to use it vs. the above approach.

[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