surfaceflinger / GL extensions cleanup

Add correct enumerants for OES_EGL_image_external to glext.h.
SurfaceFlinger now checks for the correct extension name.

Change-Id: I2ba2728a01fa2260bd086d2df4316c68f694a9b1
diff --git a/services/surfaceflinger/GLExtensions.cpp b/services/surfaceflinger/GLExtensions.cpp
index 7f4f9fc..850866a 100644
--- a/services/surfaceflinger/GLExtensions.cpp
+++ b/services/surfaceflinger/GLExtensions.cpp
@@ -86,7 +86,7 @@
         mHaveNpot = true;
     }
 
-    if (hasExtension("GL_OES_texture_external")) {
+    if (hasExtension("GL_OES_EGL_image_external")) {
         mHaveTextureExternal = true;
     } else if (strstr(mRenderer.string(), "Adreno")) {
         // hack for Adreno 200