8353230: Emoji rendering regression after JDK-8208377

[gredler]

It looks like this regression actually fits into a longer series of fixes / regressions in this area:

• JDK-4517298 fixed metrics for zero-width characters, but broke some ligatures / glyph substitutions
• JDK-7017058 fixed the ligatures / glyph substitutions, but broke some zero-width metrics
• JDK-8208377 fixed some metrics and rendering for zero-width characters, but broke some ligatures / glyph substitutions
• Now, with this PR, we aim to fix the ligatures without re-breaking zero-width metrics and display

We have two different types of use cases pulling CharToGlyphMapper in two different directions: the users who need raw, untransformed glyph info, and the users who need normalized / transformed glyph info.

It looks to me like, in the current code base, the only CharToGlyphMapper user which requires raw font data is HarfBuzz (explicitly confirmed with the HarfBuzz team here: harfbuzz/harfbuzz#5234).

The regression mechanism at play here is that the HarfBuzz font callbacks are currently providing HarfBuzz with transformed glyph info (e.g. ZWJ -> INVISIBLE_GLYPH_ID), which prevents HarfBuzz from recognizing and applying the correct font GSUB substitutions (which involve ZWJ).

In order to fix this without (yet again) breaking metrics and display behavior elsewhere, I've added two methods to CharToGlyphMapper which provide access to raw glyph info, to be used by the HarfBuzz font callbacks: charToGlyphRaw(int) and charToVariationGlyphRaw(int).

Note two intricacies related to CompositeGlyphMapper:

1. We need to be careful to only cache raw (untransformed) values, to avoid conflicts between requests for a raw version of a glyph and a transformed version of the same glyph. Another option would have been two separate caches, but I don't think that's necessary.
2. Consumers who are using CompositeGlyphMapper.SLOTMASK to check glyph slots (e.g. FontRunIterator and CTextPipe) will "see" invisible glyphs as having come from slot 0. This isn't new, and I think it's OK, but something to be aware of.

The glyph cache handling in CCharToGlyphMapper (for macOS) also requires care to avoid mixing value types.

Please also note that I'm not sure if the tweak to sunFont.c is being tested, since FFM is being used by default for HarfBuzz integration. (Is there a plan to remove the JNI version soon?)

This PR includes a self-contained regression test. It includes a small font created just for this test, which exercises the ligature / glyph substitution infrastructure. The font tests, including the new regression test, all pass locally on Linux, Windows and macOS (make test TEST="jtreg:test/jdk/java/awt/font").

Interestingly, the changes for JDK-7017058 (mentioned above) included a test (ZWJLigatureTest) which I think would have caught this last regression, but it depends on optional Windows fonts which I guess do not exist on any commonly-used test infrastructure. This should not be an issue with the new test, since it does not depend on any external fonts.

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• JDK-8353230: Emoji rendering regression after JDK-8208377 (Bug - P2)

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