In FPs, store pointers for all child slots, even nullptr

This simplifies things like ConstantOutputForConstantInput and
invokeChild. It also removes the need for child indices: generated
FPs now directly refer to their children by slot number.

Change-Id: I69bbb042d5d72d21b999256f969c467702d0774d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/302436
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
diff --git a/src/gpu/gradients/generated/GrSweepGradientLayout.cpp b/src/gpu/gradients/generated/GrSweepGradientLayout.cpp
index 2f07145..5ca21ae 100644
--- a/src/gpu/gradients/generated/GrSweepGradientLayout.cpp
+++ b/src/gpu/gradients/generated/GrSweepGradientLayout.cpp
@@ -84,6 +84,7 @@
         : INHERITED(kGrSweepGradientLayout_ClassID, src.optimizationFlags())
         , bias(src.bias)
         , scale(src.scale) {
+    this->cloneAndRegisterAllChildProcessors(src);
     this->setUsesSampleCoordsDirectly();
 }
 std::unique_ptr<GrFragmentProcessor> GrSweepGradientLayout::clone() const {