Reimplement TwoPointConicalGradient_gpu

    4.24% faster in gradient_conical_clamp_shallow_dither
    6.43% faster in gradient_conicalOutZero_clamp_hicolor
   10.41% faster in gradient_conicalOut_clamp_hicolor
   14.85% faster in gradient_conicalOutZero_clamp_3color
   16.72% faster in gradient_conicalOut_clamp
   24.30% faster in gradient_conicalOut_clamp_3color

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Android-Clang-Nexus7-GPU-Tegra3-arm-Release-All-Android,Test-Android-Clang-Nexus7-GPU-Tegra3-arm-Debug-All-Android

Bug: skia:
Change-Id: I6bc1f24c9463fc9c2acbcba7bd4d55b37ade1613
Reviewed-on: https://skia-review.googlesource.com/82161
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
5 files changed