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.h b/src/compiler/ParseHelper.h
index 8637d89..7efc23f 100644
--- a/src/compiler/ParseHelper.h
+++ b/src/compiler/ParseHelper.h
@@ -85,6 +85,7 @@
     bool nonInitErrorCheck(int line, TString& identifier, TPublicType& type, TVariable*& variable);
     bool paramErrorCheck(int line, TQualifier qualifier, TQualifier paramQualifier, TType* type);
     bool extensionErrorCheck(int line, const TString&);
+    bool supportsExtension(const char* extension);
     const TFunction* findFunction(int line, TFunction* pfnCall, bool *builtIn = 0);
     bool executeInitializer(TSourceLoc line, TString& identifier, TPublicType& pType,
                             TIntermTyped* initializer, TIntermNode*& intermNode, TVariable* variable = 0);