Initial commit of libAgl2 using Pixelflinger2 in external/mesa3d

Somewhat functional, refer to README for details.
Need to enable Android.mk to build.
It builds libGLES_android.so, which needs to replace
 the one in system/lib/egl built by libagl.

Change-Id: Iec3aaa8f3963a4185d81955cd24019eb0c4a5850
Signed-off-by: David Li <davidxli@google.com>