Fix to set correct output type when blending when we've read dst

src/gpu/GrOptDrawState.h

Index: src/gpu/GrOptDrawState.h
diff --git a/src/gpu/GrOptDrawState.h b/src/gpu/GrOptDrawState.h
index cf5737373caa44ba3134f6a7d9efb56bc52ecef4..55a5071d4179414af49eca990ec7166d934fe678 100644
--- a/src/gpu/GrOptDrawState.h
+++ b/src/gpu/GrOptDrawState.h
@@ -79,7 +79,8 @@ public:
     int numCoverageStages() const { return fFragmentStages.count() - fNumColorStages; }
     int numFragmentStages() const { return fFragmentStages.count(); }
     int numTotalStages() const {
-         return this->numFragmentStages() + (this->hasGeometryProcessor() ? 1 : 0);
+        // the + 1 at the end is for the xferProcessor which will always be present
+        return this->numFragmentStages() + (this->hasGeometryProcessor() ? 1 : 0) + 1;
     }
 
     bool hasGeometryProcessor() const { return SkToBool(fGeometryProcessor.get()); }