[cairo] Optimize horizontal/vertical gradients

[Keith Packard]

Around 23 o'clock on Feb 13, Owen Taylor wrote:

> It's possible that there should
> be some epsilon in the tests, to handle round-off errors in
> 90 degree rotations and the like, maybe something like
> 
>   fabs (p2.x - p1.x) < EPSILON * fabs (p2.y - p1.y)

That's probably a good idea.  I suggest we can find an appropriate EPSILON
by evaluating the extreme values of the two bracketing terms assuming 
the user computed them with the standard C library sin/cos functions using 
the angles -3.0 * M_PI / 2.0, -M_PI/2.0, M_PI/2.0 and 3.0 * M_PI / 2.0.

-keith


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