This avoids recomputing the span data for every dash. x11perf thinks
this is a pretty modest speedup:
832919.4 840471.1 ( 1.009) 100-pixel dashed ellipse
672353.1 680652.2 ( 1.012) 100-pixel double-dashed ellipse
13748.9 24287.9 ( 1.767) 100-pixel wide dashed ellipse
9236.3 21298.2 ( 2.306) 100-pixel wide double-dashed ellipse
But part of the reason it's so modest there is that the arcs are
relatively small (100 pixel diameter at line width 10, so ~6000 pixels)
and the dashes relatively large (30 on 20 off so ~6 dashes per
quadrant).
With larger arcs and finer dashes this is much more impressive. A fairly
trivial testcase of a single 15000x13000 arc with the default {2, 2}
dash pattern drops from ~3500 milliseconds to 10 milliseconds.
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>
0d7f05ed99