implement EGL_TEXTURE_EXTERNAL_OES in libagl

this allows the emulator to display some graphics

Change-Id: Ib4671ad70b8df598d02307f2b9c5b843421cea25
diff --git a/opengl/libagl/state.cpp b/opengl/libagl/state.cpp
index 8b4136a..90e9612 100644
--- a/opengl/libagl/state.cpp
+++ b/opengl/libagl/state.cpp
@@ -191,6 +191,9 @@
         // these need to fall through into the rasterizer
         c->rasterizer.procs.enableDisable(c, cap, enabled);
         break;
+    case GL_TEXTURE_EXTERNAL_OES:
+        c->rasterizer.procs.enableDisable(c, GL_TEXTURE_2D, enabled);
+        break;
 
     case GL_MULTISAMPLE:
     case GL_SAMPLE_ALPHA_TO_COVERAGE: