fillCoverage refactor

[guzba]

before:

svg render ........................ 11.926 ms     12.011 ms    ±0.121   x415 [simd]
svg render ........................ 17.460 ms     17.571 ms    ±0.181   x284 [no simd]

after:

svg render ........................ 11.761 ms     11.832 ms    ±0.103   x422 [simd]
svg render ........................ 15.190 ms     15.301 ms    ±0.142   x326 [no simd]