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.

drivers/SCsub

@@ -24,6 +24,7 @@ if env['xaudio2']:
 # Graphics drivers
 if (env["platform"] != "server"):
     SConscript('gles3/SCsub')
+    SConscript('gles2/SCsub')
     SConscript('gl_context/SCsub')
 else:
     SConscript('dummy/SCsub')