Fix end of arc condition in arc rendering

[Ben Lippmeier]

With this program:

import Graphics.Gloss
main    
= animate (InWindow "Foo" (800,800) (0,0)) black 
$ \t -> pictures 
       [ color white $ ThickArc (5 * t+ 0.01) (10 * t+ 0.1) 200 100
       , color red   $ ThickArc (5 * t+ 0.01) (10 * t+ 0.1) 250 10
       , color red   $ ThickArc (5 * t+ 0.01) (10 * t+ 0.1) 150 10 ]

The arc ends aren't at the right angle, depending on the value for the total arc length. I expect this is an interaction between the number of draw steps and the total length.

[Ben Lippmeier]

End of arc condition is wrong