Update invariant output computation for various texture effects.

src/gpu/effects/GrTextureDomain.cpp

Index: src/gpu/effects/GrTextureDomain.cpp
diff --git a/src/gpu/effects/GrTextureDomain.cpp b/src/gpu/effects/GrTextureDomain.cpp
index db3ef2cd98715064211c55dfefa6ac66576fc4a0..de552c0ac9665472492c3ff63cf82787837d18ed 100644
--- a/src/gpu/effects/GrTextureDomain.cpp
+++ b/src/gpu/effects/GrTextureDomain.cpp
@@ -271,7 +271,11 @@ bool GrTextureDomainEffect::onIsEqual(const GrFragmentProcessor& sBase) const {
 
 void GrTextureDomainEffect::onComputeInvariantOutput(GrInvariantOutput* inout) const {
     if (GrTextureDomain::kDecal_Mode == fTextureDomain.mode()) { // TODO: helper
-        inout->mulByUnknownColor();
+        if (GrPixelConfigIsAlphaOnly(this->texture(0)->config())) {
+            inout->mulByUnknownAlpha();
+        } else {
+            inout->mulByUnknownColor();
+        }
     } else {
         this->updateInvariantOutputForModulation(inout);
     }