2pt conical stage for focal-pt-on-edge case

When the focal point is on the edge of the end circle, the quadratic
equation devolves to linear.  Add a stage to handle this case.

As a complication, this case can produce "degenerate" values:

 1) t == NaN
 2) R(t) < 0

For these, we're supposed to draw transparent black - which means
overwriting the color from the gradient stage.  To support this, build
a 0/1 vector mask in the context, and apply it post-gradient-stage.

Change-Id: Ice4e3243abfd8c784bb810f6c310aed7a4ac7dc8
Reviewed-on: https://skia-review.googlesource.com/21111
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@google.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/src/shaders/gradients/SkGradientShaderPriv.h b/src/shaders/gradients/SkGradientShaderPriv.h
index 3322982708..6e6e7df074 100644
--- a/src/shaders/gradients/SkGradientShaderPriv.h
+++ b/src/shaders/gradients/SkGradientShaderPriv.h
@@ -239,7 +239,10 @@ protected:
 
     virtual bool adjustMatrixAndAppendStages(SkArenaAlloc* alloc,
                                              SkMatrix* matrix,
-                                             SkRasterPipeline* p) const { return false; }
+                                             SkRasterPipeline* tPipeline,
+                                             SkRasterPipeline* postPipeline) const {
+        return false;
+    }
 
     template <typename T, typename... Args>
     static Context* CheckedMakeContext(SkArenaAlloc* alloc, Args&&... args) {