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/ParseHelper.cpp b/src/compiler/ParseHelper.cpp
index 3f0836d..e2141c6 100644
--- a/src/compiler/ParseHelper.cpp
+++ b/src/compiler/ParseHelper.cpp
@@ -943,6 +943,12 @@
     return false;
 }
 
+bool TParseContext::supportsExtension(const char* extension)
+{
+    TExtensionBehavior::const_iterator iter = extensionBehavior.find(extension);
+    return (iter != extensionBehavior.end());
+}
+
 /////////////////////////////////////////////////////////////////////////////////
 //
 // Non-Errors.