mesa: Bring in new mesa sub-statetracker.

Some code cleanup is still in order.
diff --git a/src/mesa/SConscript b/src/mesa/SConscript
index 89b98b3..f4e138c 100644
--- a/src/mesa/SConscript
+++ b/src/mesa/SConscript
@@ -337,3 +337,6 @@
 			source = glapi_sources,
 		)
 		Export('glapi')
+
+	if platform == 'windows':
+		SConscript('state_tracker/wgl/SConscript')