Update invariant output computation for various texture effects.

src/gpu/effects/GrSingleTextureEffect.h

Index: src/gpu/effects/GrSingleTextureEffect.h
diff --git a/src/gpu/effects/GrSingleTextureEffect.h b/src/gpu/effects/GrSingleTextureEffect.h
index efdf25555259d315cf66970f18daca472d4de8ce..6fd6819e92ac6d688da0659fb15d6c04fffb096b 100644
--- a/src/gpu/effects/GrSingleTextureEffect.h
+++ b/src/gpu/effects/GrSingleTextureEffect.h
@@ -40,7 +40,9 @@ protected:
      * texture.
      */
     void updateInvariantOutputForModulation(GrInvariantOutput* inout) const {
-        if (GrPixelConfigIsOpaque(this->texture(0)->config())) {
+        if (GrPixelConfigIsAlphaOnly(this->texture(0)->config())) {
+            inout->mulByUnknownAlpha();
+        } else if (GrPixelConfigIsOpaque(this->texture(0)->config())) {
             inout->mulByUnknownOpaqueColor();
         } else {
             inout->mulByUnknownColor();