Add isSingleComponent bool to getConstantColorComponent

Initial step to allowing effects to use/output 1 or 4 color/coverage components. This cl doesn't change any current logic and all effects still assume they are working with 4 components.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/3b8af078281a5a20f951b9fd84f38d92b8f6217b

Review URL: https://codereview.chromium.org/608253002
diff --git a/src/gpu/effects/GrCustomCoordsTextureEffect.cpp b/src/gpu/effects/GrCustomCoordsTextureEffect.cpp
index b9794c1..7362ba9 100644
--- a/src/gpu/effects/GrCustomCoordsTextureEffect.cpp
+++ b/src/gpu/effects/GrCustomCoordsTextureEffect.cpp
@@ -72,14 +72,13 @@
     return fTextureAccess == cte.fTextureAccess;
 }
 
-void GrCustomCoordsTextureEffect::getConstantColorComponents(GrColor* color,
-                                                             uint32_t* validFlags) const {
-    if ((*validFlags & kA_GrColorComponentFlag) && 0xFF == GrColorUnpackA(*color) &&
-        GrPixelConfigIsOpaque(this->texture(0)->config())) {
-        *validFlags = kA_GrColorComponentFlag;
+void GrCustomCoordsTextureEffect::onComputeInvariantOutput(InvariantOutput* inout) const {
+    if (inout->isOpaque() && GrPixelConfigIsOpaque(this->texture(0)->config())) {
+        inout->fValidFlags = kA_GrColorComponentFlag;
     } else {
-        *validFlags = 0;
+        inout->fValidFlags = 0;
     }
+    inout->fIsSingleComponent = false;
 }
 
 const GrBackendGeometryProcessorFactory& GrCustomCoordsTextureEffect::getFactory() const {