Try harder to only redraw the strokes that have actually changed.
Try harder to only redraw the strokes that have actually changed.

I think this should improve redisplay performance by 30-40% (when
changing the buffer), but I don't want to affect the result by
measuring it.

swflint/McCLIM,onixie/mcclim,slyrus/mcclim,swflint/McCLIM,antifuchs/mcclim,onixie/mcclim,antifuchs/mcclim,onixie/mcclim,swflint/McCLIM,slyrus/mcclim