See if it is possible to dynamically load OpenGL ES 3, and fallback to 2 if not available on the device

[jgranick]

For long-term backward compatibility, it would be ideal to have support for referencing OpenGL ES 3.0 APIs dynamically, attempting to load the proper library at runtime, and falling back to OpenGL ES 2.0 if it cannot be found.

We should add a -Dgles2 define if we get this behavior to work, so that a developer can force an OpenGL ES 2.0 context if they would prefer to not allow OpenGL ES 3.0 in their application, otherwise it should be used by default.