fix [2793164] Spam 2x/second with TOT master in SurfaceFlinger

Make sure to not use GL_TEXTURE_EXTERNAL when it's not supported
by the GL. The error was harmless, but annoying.

Change-Id: I571a9a9b05d35da51420950a6a6e95629067efd0
diff --git a/libs/surfaceflinger/Layer.h b/libs/surfaceflinger/Layer.h
index dcb27a0..e1d283b 100644
--- a/libs/surfaceflinger/Layer.h
+++ b/libs/surfaceflinger/Layer.h
@@ -37,9 +37,10 @@
 
 // ---------------------------------------------------------------------------
 
-class Client;
-class UserClient;
 class FreezeLock;
+class Client;
+class GLExtensions;
+class UserClient;
 
 // ---------------------------------------------------------------------------
 
@@ -206,6 +207,7 @@
     // constants
     sp<Surface> mSurface;
     PixelFormat mFormat;
+    const GLExtensions& mGLExtensions;
     bool mNeedsBlending;
     bool mNeedsDithering;