Disable GL_OES_standard_derivatives when ps_2_x or later is not available.

BUG=392
Review URL: https://codereview.appspot.com/7027051


Author:    bsalomon@google.com

(manual merge from master by daniel@transgaming.com)

git-svn-id: https://angleproject.googlecode.com/svn/branches/dx11proto@1704 736b8ea6-26fd-11df-bfd4-992fa37f6226
diff --git a/src/libGLESv2/Context.h b/src/libGLESv2/Context.h
index eb9c75c..6c52f8a 100644
--- a/src/libGLESv2/Context.h
+++ b/src/libGLESv2/Context.h
@@ -409,6 +409,7 @@
     bool supportsNonPower2Texture() const;
     bool supportsInstancing() const;
     bool supportsTextureFilterAnisotropy() const;
+    bool supportsDerivativeInstructions() const;
 
     bool getCurrentReadFormatType(GLenum *format, GLenum *type);
 
@@ -515,6 +516,7 @@
     bool mSupportsDepthTextures;
     bool mSupports32bitIndices;
     bool mSupportsTextureFilterAnisotropy;
+    bool mSupportsDerivativeInstructions;
     int mNumCompressedTextureFormats;
 
     ResourceManager *mResourceManager;