Impeller stroke simplify dot miter computations

[flar]

Simplifies some of the repetitive computations in the cap and join operations in the Impeller stroking code. Precomputes the data and consolidates it into a structure for simplified passing to cap and join procs.

Local testing shows a 20% improvement in the Flutter default Cap::kButt/Join::kMiter attribute combination.

[flutter-dashboard[bot]]

This pull request has been changed to a draft. The currently pending flutter-gold status will not be able to resolve until a new commit is pushed or the change is marked ready for review again.

[goderbauer]

I spoke to @flar last week and he said this is still on his radar.