Add gpu implementation of SkAvoidXfermode

gm/avoidxfermode.cpp

Index: gm/avoidxfermode.cpp
diff --git a/gm/avoidxfermode.cpp b/gm/avoidxfermode.cpp
index 0d681cb143c61554959f465cb15d770262411dec..a015b6bbe7085e8c01fcf3719ce06523dfe29bb2 100644
--- a/gm/avoidxfermode.cpp
+++ b/gm/avoidxfermode.cpp
@@ -67,9 +67,9 @@ protected:
 
         r.offsetTo(0.0f, 64);
 
-        // LL corner: replace red with blue with a loose tolerance
+        // LL corner: replace red with transparent blue with a loose tolerance
         SkPaint p3;
-        p3.setColor(SK_ColorBLUE);
+        p3.setColor(0x7F0000FF);
         p3.setXfermode(SkAvoidXfermode::Create(SK_ColorRED,
                                                250,
                                                SkAvoidXfermode::kTargetColor_Mode))->unref();