Leave a character the document font has in that font when substituting (#76)

[jakejackson1]

Part of #76. Split out of #299, where it was the second commit. It fixes the CBDT, sbix and COLRv0 fonts already merged, so it doesn't need to wait for COLRv1.

Substitution no longer moves a character the document font has into a backup font.

What was wrong

A run moved to a backup font carried the spaces inside it along ($flag == 1 && $char == 32 in SubstituteCharsMB()). So a space between two emoji was the emoji font's own. Noto Color Emoji's space is as wide as an emoji, which left an emoji-sized blank after every space between emoji.

What changes

• A space now ends the run, and stays in the document font.
• The Plane 2 branch had the same clause, but it never applied: that branch also requires the character to be above U+1FFFF.
• The core-font path, SubstituteCharsNonCore(), carried spaces the same way and gets the same rule.
• That path also read fonttrans[$bsf] for a backup font that isn't a core font's name, which warned on every such font. The read is now guarded.
• The CBDT, sbix and COLRv0 snapshots are refreshed. Their spaces between emoji are DejaVu's now.

What it costs

Every substituted run now ends at a space, so a run of words is one span per word, not one per paragraph.

• The cbdt snapshot has 99 Tf where it had 17.
• Each snapshot grows about 9 KB. DejaVu is now embedded to draw the spaces, where those documents otherwise had only emoji.
• The same applies to non-emoji scripts, e.g. Korean text in a DejaVu document: its spaces are now DejaVu's.
• To pay for the extra passes, SubstituteCharsMB() finds emoji as its scan reaches them (Emoji::clusterAt(), now public), instead of finding every emoji in the rest of the text on every pass. The output is the same. On a 2000-word paragraph that is 1376 ms → 292 ms for emoji and 2262 ms → 573 ms for Korean.

Tests

EmojiSubstitutionTest asserts the space splits the run, for a TrueType document font and for Helvetica.

Merging with #299

#299's color-emoji-colrv1 snapshot and #300's color-emoji-svg were made without this change. Whichever of #299 and this PR merges second needs a rebase and composer snapshot:update color-emoji-colrv1 (and color-emoji-svg on #300).

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