Alter SkXfermode's asFragmentProcessor & asXPFactory contracts

src/gpu/SkGr.cpp

Index: src/gpu/SkGr.cpp
diff --git a/src/gpu/SkGr.cpp b/src/gpu/SkGr.cpp
index e4dc467e7f8b9ce2ccd27d5957f505371a515063..e7e34cda048007a49292bb85c50ea3efe6cf391a 100644
--- a/src/gpu/SkGr.cpp
+++ b/src/gpu/SkGr.cpp
@@ -489,10 +489,14 @@ static inline bool skpaint_to_grpaint_impl(GrContext* context,
         }
     }
 
-    SkXfermode* mode = skPaint.getXfermode();
-    GrXPFactory* xpFactory = nullptr;
-    SkXfermode::AsXPFactory(mode, &xpFactory);
-    SkSafeUnref(grPaint->setXPFactory(xpFactory));
+    // When the xfermode is null on the SkPaint (meaning kSrcOver) we need the XPFactory field on
+    // the GrPaint to also be null (also kSrcOver).
+    SkASSERT(!grPaint->getXPFactory());
+    SkXfermode* xfermode = skPaint.getXfermode();
+    if (xfermode) {
+        // SafeUnref b.c. kSrcOver_Mode will return a null factory

[egdaniel, 2016/02/08 16:09:03]

Nit: it is not that kSrcOver_Mode will return a null factory (I actually don't
think it will), but just the general fact that we allow any xfermode to return
null and we will simply fall back to srcOver in that case.

[robertphillips, 2016/02/08 16:24:15]

Done.

+        SkSafeUnref(grPaint->setXPFactory(xfermode->asXPFactory()));
+    }
 
 #ifndef SK_IGNORE_GPU_DITHER
     if (skPaint.isDither() && grPaint->numColorFragmentProcessors() > 0) {