Update invariant output computation for various texture effects.

src/effects/SkAlphaThresholdFilter.cpp

Index: src/effects/SkAlphaThresholdFilter.cpp
diff --git a/src/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index bff877280c5689e1bd4371a3d502c856d141b196..c01dd975d0641b1fe6d4de06ed6d209415d807ef 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -229,7 +229,9 @@ bool AlphaThresholdEffect::onIsEqual(const GrFragmentProcessor& sBase) const {
 }
 
 void AlphaThresholdEffect::onComputeInvariantOutput(GrInvariantOutput* inout) const {
-    if (GrPixelConfigIsOpaque(this->texture(0)->config()) && fOuterThreshold >= 1.f) {
+    if (GrPixelConfigIsAlphaOnly(this->texture(0)->config())) {
+        inout->mulByUnknownAlpha();
+    } else if (GrPixelConfigIsOpaque(this->texture(0)->config()) && fOuterThreshold >= 1.f) {
         inout->mulByUnknownOpaqueColor();
     } else {
         inout->mulByUnknownColor();