Remove dead gradient perspective code

The cool bit here is that Sk4fLinearGradient is now always fast, and never calls the slow
base-class impl.  This means we can rip out Sk4fGradientBase::{mapTs, shadeSpan4f, etc}.

Change-Id: Id3788bc810873b2a209f66efa4187c84b3397e2f
Reviewed-on: https://skia-review.googlesource.com/22366
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/src/shaders/gradients/SkGradientShader.cpp b/src/shaders/gradients/SkGradientShader.cpp
index 0619ef0..6377018 100644
--- a/src/shaders/gradients/SkGradientShader.cpp
+++ b/src/shaders/gradients/SkGradientShader.cpp
@@ -534,12 +534,9 @@
     const SkMatrix& inverse = this->getTotalInverse();
 
     fDstToIndex.setConcat(shader.fPtsToUnit, inverse);
+    SkASSERT(!fDstToIndex.hasPerspective());
 
     fDstToIndexProc = fDstToIndex.getMapXYProc();
-    fDstToIndexClass = (uint8_t)SkShaderBase::Context::ComputeMatrixClass(fDstToIndex);
-
-    // TODO: remove all perspective-related gradient code
-    SkASSERT(fDstToIndexClass == kLinear_MatrixClass);
 
     // now convert our colors in to PMColors
     unsigned paintAlpha = this->getPaintAlpha();