[cairo] Faster Radial Gradients patch

[David Turner]

Hello,

  here's a small patchset that speeds up radial gradient rendering up to 1.48x.
  the trick is simply to optimize the computations through simplifications and
  forward differencing.

    http://david.freetype.org/cairo/faster-radial-gradients-1.patchset

  other tricks like using fixed point arithmetics, or approximating the square
  root extraction per pixel may be used in the future.

  note that the patch also introduces support for degenerate cases like 
  when the two circles are identical, or when they touch on a single point
  of contact (respectively A == 0 && B == 0  and  A == 0 && B != 0)

Regards,

- David Turner
- The FreeType Project  (www.freetype.org)