pattern::tests::kept_tiles_count_the_images_they_could_not_decode
draws a pattern whose art is an image three times and expects one tile
to be rasterized. A tile of art with an image is kept only while
FontDb::global().generation() is the one it was drawn at (Deps::current
in pattern.rs). The render tests that hold adding_fonts() add fonts,
which increases it (tests/fontchange.rs, tests/typeedges.rs,
tests/alternate.rs, tests/colrglyphs.rs and tests/bitmapglyphs.rs).
When one of them added a font between the frames, the next frame drew
the tile again and the test failed with left 2 or 3, right 1. Run with
the tests in tests/fontchange.rs and tests/typeedges.rs, it failed in
23 of 30 runs on main.
The test now holds fonts_steady() and takes its counts from
with_fonts_unchanged(), as the other render tests that count kept work
do. Each run builds a new document, so its first frame does not find a
tile from an earlier run. Run with the same tests, it passed in all 30
runs.
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