Improvements for circluar blurs in GPU backend.

Use half plane approximation for small blur of a large circle. This reduces
the number of textures used and also avoids numerical issues with large
circle radii.

Make GrCircleGradientEffect use fixed sized textures.
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2242973002

Review-Url: https://codereview.chromium.org/2242973002
2 files changed