remove TODO in SkGradientShader.cpp

As we have factored things, we _do_ need to map y.  This matrix maps us
back from dst to src space now, but not all the way to t.
We let the subclass inject its own logic for x,y -> t.

(Linear gradients do actually still do this by modifying the first
matrix rather than appending another matrix for x,y -> t.  But in
general we cannot not map y here anymore.)

Change-Id: I2f9a87c7b510269a8b218a0247cbff416fe8d6c9
Reviewed-on: https://skia-review.googlesource.com/16495
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>

src/effects/gradients/SkGradientShader.cpp

@@ -364,7 +364,6 @@ bool SkGradientShaderBase::onAppendStages(
 
     auto* m = alloc->makeArrayDefault<float>(9);
     if (matrix.asAffine(m)) {
-        // TODO: mapping y is not needed; split the matrix stages to save some math?
         pipeline->append(SkRasterPipeline::matrix_2x3, m);
     } else {
         matrix.get9(m);