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/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index f199ca9..dc241d4 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -917,7 +917,7 @@
         glVertexPointer(2, GL_SHORT, 0, vertices);
         glTexCoordPointer(2, GL_SHORT, 0, tcoords);
         glEnableClientState(GL_TEXTURE_COORD_ARRAY);
-#if defined(GL_OES_texture_external)
+#if defined(GL_OES_EGL_image_external)
         if (GLExtensions::getInstance().haveTextureExternal()) {
             glDisable(GL_TEXTURE_EXTERNAL_OES);
         }