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>