Only allow zero indexes for gl_FragData when the draw buffers extension is disabled.

TRAC #23509

Signed-off-by: Shannon Woods
Signed-off-by: Nicolas Capens
Authored-by: Jamie Madill
diff --git a/src/compiler/ParseHelper.h b/src/compiler/ParseHelper.h
index 79abe43..51951e6 100644
--- a/src/compiler/ParseHelper.h
+++ b/src/compiler/ParseHelper.h
@@ -111,6 +111,7 @@
     const TPragma& pragma() const { return directiveHandler.pragma(); }
     const TExtensionBehavior& extensionBehavior() const { return directiveHandler.extensionBehavior(); }
     bool supportsExtension(const char* extension);
+    bool isExtensionEnabled(const char* extension) const;
     void handleExtensionDirective(const TSourceLoc& loc, const char* extName, const char* behavior);
     void handlePragmaDirective(const TSourceLoc& loc, const char* name, const char* value);