Add AlphaThreshold filter.

This is based on the Bitmap Alpha Threshold filter, and will be used by Chromium
to implement the window shape API.

R=bsalomon@chromium.org, wez@chromium.org, bsalomon@google.com, reed@google.com

Author: zork@chromium.org

Review URL: https://codereview.chromium.org/115633002

git-svn-id: http://skia.googlecode.com/svn/trunk@12935 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tests/GLProgramsTest.cpp b/tests/GLProgramsTest.cpp
index cbf00fd..a1dbea8 100644
--- a/tests/GLProgramsTest.cpp
+++ b/tests/GLProgramsTest.cpp
@@ -251,6 +251,7 @@
 // in the unit that could pass pointers to functions from the unit out to other translation units!
 // We force some of the effects that would otherwise be discarded to link here.
 
+#include "SkAlphaThresholdFilter.h"
 #include "SkLightingImageFilter.h"
 #include "SkMagnifierImageFilter.h"
 #include "SkColorMatrixFilter.h"
@@ -259,6 +260,7 @@
 
 void forceLinking() {
     SkLightingImageFilter::CreateDistantLitDiffuse(SkPoint3(0,0,0), 0, 0, 0);
+    SkAlphaThresholdFilter::Create(SkRegion(), .5, .5);
     SkMagnifierImageFilter mag(SkRect::MakeWH(SK_Scalar1, SK_Scalar1), SK_Scalar1);
     GrConfigConversionEffect::Create(NULL,
                                      false,