Add 4x4 downsample filter with 4 bilinear texture reads, use for ssaa.

Review URL: http://codereview.appspot.com/4483042/

git-svn-id: http://skia.googlecode.com/svn/trunk@1250 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gpu/include/GrContext_impl.h b/gpu/include/GrContext_impl.h
index 0fa3b8d..fae4e92 100644
--- a/gpu/include/GrContext_impl.h
+++ b/gpu/include/GrContext_impl.h
@@ -21,6 +21,11 @@
     OffscreenRecord() { fEntry0 = NULL; fEntry1 = NULL; }
     ~OffscreenRecord() { GrAssert(NULL == fEntry0 && NULL == fEntry1); }
 
+    enum Downsample {
+        k4x4TwoPass_Downsample,
+        k4x4SinglePass_Downsample,
+        kFSAA_Downsample
+    }                              fDownsample;
     GrTextureEntry*                fEntry0;
     GrTextureEntry*                fEntry1;
     GrDrawTarget::SavedDrawState   fSavedState;