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/LayerBlur.cpp b/services/surfaceflinger/LayerBlur.cpp
index 2ee21b9..4cfcfe3 100644
--- a/services/surfaceflinger/LayerBlur.cpp
+++ b/services/surfaceflinger/LayerBlur.cpp
@@ -146,7 +146,7 @@
     Region::const_iterator it = clip.begin();
     Region::const_iterator const end = clip.end();
     if (it != end) {
-#if defined(GL_OES_texture_external)
+#if defined(GL_OES_EGL_image_external)
         if (GLExtensions::getInstance().haveTextureExternal()) {
             glDisable(GL_TEXTURE_EXTERNAL_OES);
         }