This change takes polygon from 126k pixels per second fill to 240k pps fill
on a reference 5 point star 50x66px polygon, updating both location and shape
at 10hz. Tested on an m4 express feather.
As a curiosity, the flat-out fill rate of a shape whose get_pixel is `return 0;`
fills just shy of 375k pixels per second.
4bbb80e31a