Add support to Raspberry Pi

[AaronRays468]

[JayFoxRox]

I'd assume this already works if you have a GL implementation (such as MESA SW, possibly something like nanogl).

As we are slowly progressing to librw this might become easier to do as we could add an unofficial GLES backend to librw in a rather clean API abstraction.

[valpackett]

Mesa VC4 actually supports GL 2.1, not just GLES

[kargaroc]

That's pretty nice, though I don't like all the workarounds needed to get desktop GL working on a device which is only physically capable of GLES2. A GLES backend would still be helpful to improve performance and eliminate bugs.

Also, a raspberry pi build of this would be cool.