Optimise the size of Vello (Classic) test snapshots using oxipng

[DJMcNab]

The size of these screenshots was the main reason we ended up using LFS for storing snapshots. These have now been reduced significantly by using oxipng, as in Vello Sparse Tests:

.../vello/vello_tests/snapshots/big_bitmap.png: 8.87 kB (-5.70 kB)
.../vello/vello_tests/snapshots/big_bitmap_apple.png: 13.30 kB (-5.06 kB)
.../vello/vello_tests/snapshots/big_colr.png: 10.14 kB (-5.87 kB)
.../vello/vello_tests/snapshots/bitmap_undef.png: 68 B (-221 B)
.../vello/vello_tests/snapshots/blurred_rounded_rect.png: 19.25 kB (-102.38 kB)
.../vello/vello_tests/snapshots/colr_undef.png: 68 B (-221 B)
.../vello/vello_tests/snapshots/deep_blend.png: 236 B (-6.76 kB)
.../vello/vello_tests/snapshots/fill_types.png: 27.77 kB (-54.34 kB)
.../vello/vello_tests/snapshots/funky_paths.png: 2.31 kB (-15.68 kB)
.../vello/vello_tests/snapshots/gradient_extend.png: 11.23 kB (-44.17 kB)
.../vello/vello_tests/snapshots/image_extend_modes_bilinear.png: 18.81 kB (-102.47 kB)
.../vello/vello_tests/snapshots/image_extend_modes_nearest_neighbor.png: 911 B (-19.11 kB)
.../vello/vello_tests/snapshots/image_sampling.png: 17.14 kB (-66.48 kB)
.../vello/vello_tests/snapshots/integer_translation.png: 657 B (-2.52 kB)
.../vello/vello_tests/snapshots/little_bitmap.png: 759 B (-705 B)
.../vello/vello_tests/snapshots/little_colr.png: 1.35 kB (-599 B)
.../vello/vello_tests/snapshots/longpathdash_butt.png: 13.62 kB (-28.81 kB)
.../vello/vello_tests/snapshots/many_clips.png: 14.47 kB (-11.04 kB)
.../vello/vello_tests/snapshots/non_integer_translation.png: 657 B (-2.52 kB)
.../vello/vello_tests/snapshots/rounded_rectangle_watertight.png: 183 B (-638 B)
.../vello/vello_tests/snapshots/scaled_hinted.png: 1.01 kB (-4.71 kB)
.../vello/vello_tests/snapshots/simple_hinted.png: 647 B (-2.48 kB)
.../vello/vello_tests/snapshots/smoke/data_image_roundtrip.png: 602 B (-59 B)
.../vello/vello_tests/snapshots/smoke/filled_circle.png: 149 B (-339 B)
.../vello/vello_tests/snapshots/smoke/filled_square.png: 82 B (-244 B)
.../vello/vello_tests/snapshots/smoke/layer_size.png: 109 B (-70 B)
.../vello/vello_tests/snapshots/smoke/two_emoji.png: 2.29 kB (-783 B)
.../vello/vello_tests/snapshots/splash.png: 49.78 kB (-50.08 kB)
.../vello/vello_tests/snapshots/stroke_styles.png: 42.30 kB (-53.71 kB)
.../vello/vello_tests/snapshots/stroke_styles_non_uniform.png: 42.73 kB (-47.17 kB)
.../vello/vello_tests/snapshots/stroke_styles_skew.png: 44.08 kB (-55.96 kB)
.../vello/vello_tests/snapshots/tricky_strokes.png: 17.37 kB (-17.36 kB)
----------------------------
Total size: 362.96 kB (-708.25 kB)

This does not move us away from LFS. I think there are some reasonable arguments for doing that now (primarily to allow using jujitsu). But I'm not planning on pushing for that until we have another need for it.