Make invokeChild default to fInputColor, rather than half4(1)

By extension, this changes the default behavior of sample() in .fp
files. This lets us remove the explicit fInputColor/sk_InColor
arguments everywhere. Most sites that were using the default no
longer care what's passed, as the child is known to be a texture
effect that will ignore the input color. The few remaining sites
now explicitly pass half4(1) when necessary.

Change-Id: Ie4691b049f905d098e9befe8bd07706a496f2968
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303356
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
diff --git a/src/gpu/glsl/GrGLSLFragmentProcessor.cpp b/src/gpu/glsl/GrGLSLFragmentProcessor.cpp
index 01900b2..4be5d82 100644
--- a/src/gpu/glsl/GrGLSLFragmentProcessor.cpp
+++ b/src/gpu/glsl/GrGLSLFragmentProcessor.cpp
@@ -43,7 +43,7 @@
 SkString GrGLSLFragmentProcessor::invokeChild(int childIndex, const char* inputColor,
                                               EmitArgs& args, SkSL::String skslCoords) {
     if (!inputColor) {
-        inputColor = "half4(1)";
+        inputColor = args.fInputColor;
     }
 
     SkASSERT(childIndex >= 0);
@@ -78,7 +78,7 @@
                                                         EmitArgs& args,
                                                         SkSL::String skslMatrix) {
     if (!inputColor) {
-        inputColor = "half4(1)";
+        inputColor = args.fInputColor;
     }
 
     SkASSERT(childIndex >= 0);