Update blurred rrect mask filter creation method to also handle caching

The caching of the mask will also be required for the vertex attribute path so I moved that into the helper. The parameters computed by ComputeBlurredRRectParams will be needed to construct the coverage geometry by the vertex attribute path so I moved that out of the helper.

This is split out of https://codereview.chromium.org/2245653002/ (Start using vertex attributes for nine-patch blurred rrect draws)

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2249463002

Review-Url: https://codereview.chromium.org/2249463002
1 file changed