Coverage counting path renderer

Initial implementation of a GPU path renderer that draws antialiased
paths by counting coverage in an offscreen buffer.

Initially disabled until it has had time to soak.

Bug: skia:
Change-Id: I003d8cfdf8dc62641581b5ea2dc4f0aa00108df6
Reviewed-on: https://skia-review.googlesource.com/21541
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/gn/samples.gni b/gn/samples.gni
index f388e79..58f8972 100644
--- a/gn/samples.gni
+++ b/gn/samples.gni
@@ -26,6 +26,7 @@
   "$_samplecode/SampleBigGradient.cpp",
   "$_samplecode/SampleBitmapRect.cpp",
   "$_samplecode/SampleBlur.cpp",
+  "$_samplecode/SampleCCPRGeometry.cpp",
   "$_samplecode/SampleCamera.cpp",
   "$_samplecode/SampleChart.cpp",
   "$_samplecode/SampleCircle.cpp",