Add an option to support for GL_OES_EGL_image_external.

Comes with this extension is the new sampler type samplerExternalOES.

ANGLEBUG=175
TEST=compile the attached shader file
Review URL: http://codereview.appspot.com/4809076

git-svn-id: https://angleproject.googlecode.com/svn/trunk@728 736b8ea6-26fd-11df-bfd4-992fa37f6226
diff --git a/src/compiler/ShaderLang.cpp b/src/compiler/ShaderLang.cpp
index ab8799c..65a4d9b 100644
--- a/src/compiler/ShaderLang.cpp
+++ b/src/compiler/ShaderLang.cpp
@@ -104,6 +104,7 @@
 
     // Extensions.
     resources->OES_standard_derivatives = 0;
+    resources->OES_EGL_image_external = 0;
 }
 
 //