add GLES 2 renderer for 2D

This commit adds a new rendering backend, GLES2, and adds a
project setting to enable it.

Currently this backend can only be used on the X11 platform,
but integrating into other platforms is planned.

modules/mobile_vr/config.py

@@ -1,6 +1,6 @@
 def can_build(platform):
     # should probably change this to only be true on iOS and Android
-    return True
+    return False
 
 def configure(env):
     pass