Optimize gradient computations

We update the test suite reference images where needed, (pdiff
avoided a few, but most still needed updating). We take advantage
of the need for new reference images to shrink some of the giant
tests to speed them up a bit.

This optimization provides a 2x improvement in linear gradient
generation performance (numbers from an x86 laptop):

image-rgb    paint_linear_rgba_source-512 26.13 -> 11.13: 2.35x speedup
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image-rgb    paint_linear_rgba_source-256  6.47 ->  2.76: 2.34x speedup
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image-rgba      paint_linear_rgb_over-256  6.51 ->  2.86: 2.28x speedup
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image-rgb      paint_linear_rgba_over-512 28.62 -> 13.70: 2.09x speedup
█▏
image-rgba       fill_linear_rgb_over-256  3.24 ->  1.94: 1.66x speedup
▋
image-rgb     stroke_linear_rgba_over-256  5.68 ->  4.10: 1.39x speedup
▍

1 files changed, 0 insertions, 0 deletions

diff --git a/test/trap-clip-ref.png b/test/trap-clip-ref.png
index 3ce2431d2..93302976b 100644
--- a/test/trap-clip-ref.png
+++ b/test/trap-clip-ref.png