[cairo] Optimize horizontal/vertical gradients

[Owen Taylor]

Owen Taylor wrote:
> Quick 30 second thought that makes a big difference for pure
> vertical/horizontal gradients. 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)

Turns out the patch was pretty essentially broken because it
was ignoring the pattern matrix ... p1/p2 are in pattern space,
not device space. Rather than inverse transforming the points
to device space and trying to detect vertical/horizontal gradients
with some epsilon, I took the alternate approach of computing
the gradient position at all four corners of the drawn area.
If the two positions on the top are the identical, it's a vertical
pattern. If the two positions on the left are identical, it's
a horizontal pattern.

Since the gradient position is handled as a fixed point number
when computing the gradient image, a simple equality test
works OK.

You could imagine more complicated algorithms that did things
like noticing that the area being drawn was completely outside
the [0,1] range and could be drawn with a constant color,
but I think this patch should catch the common cases.

There also is some cleanup of the linear gradient computation
in the patch. I'm sure the way it was written before would make
sense to me if there was a SVG diagram embedded in a comment ;-)
but it seemed to me it was introducing trig functions for no
good reason, so I rewrote it to remove them and added a short
comment.

It probably would make sense to rename various variables to match
the comment and the current logic, but I haven't yet done so
so as to keep the patch more reviewable.

I couldn't figure out a good way to share the gradient computation
logic between _cairo_linear_pattern_classify _and 
cairo_image_data_set_linear so it's just duplicated.

A short test case for test/ is included.

Regards,
					Owen
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