Add GM for filtering bug

For SkFilterQuality we get:

High   - repros for GPU
Medium - repros for both!
Low    - repros for both!
None   - doesn't repro

For AA quality (with filter quality fixed at High) we get:

AA     - repros for GPU
BW     - repros for GPU

BUG=673261

Change-Id: Ibf0644352bfa9d9c0e2d166e396ce9e9799b6d9d
Reviewed-on: https://skia-review.googlesource.com/6187
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
diff --git a/gn/gm.gni b/gn/gm.gni
index e73c1a5..a8e0eb4 100644
--- a/gn/gm.gni
+++ b/gn/gm.gni
@@ -118,6 +118,7 @@
   "$_gm/filltypes.cpp",
   "$_gm/filltypespersp.cpp",
   "$_gm/filterbitmap.cpp",
+  "$_gm/filterbug.cpp",
   "$_gm/filterfastbounds.cpp",
   "$_gm/filterindiabox.cpp",
   "$_gm/fontcache.cpp",