[cairo] perf: long dashed lines

[Vladimir Vukicevic]

Here's one patch that yields a decent speed win... _compute_face was  
doing a lot of floating point math on values that are constant for any  
given segment; this pulls those computations up out of the loops and  
just passes the values down.  Note that there are subtle changes in  
the test output with this; this is, I believe, due to the change in  
how some values are calculated in _cairo_stroker_line_to_dashed  
(particularly dx2, dy2 in the loop).

I don't have cairo-perf runs to post, I need to get to a machine  
without speedstep to do that.  The instruction count reduction is  
around 30%.

     - Vlad