SDL2/MetalANGLE support for iOS


I found out a wonderful repo that is currently building a OpenGL ES to Metal API translation layer on top of ANGLE. (kakashidinho/metalangle)

It will be nice to have that one integrated into SDL2 when Apple is definitely going to remove support for OpenGLES.
I did a porting for SDL2 in order to already use and test MetalANGLE:

Use case: