Don't canonicalize '@' to '-' in language tags

Fixes #1406.

Geeza Pro for example, relies on that for fancy mark positioning.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1405

[use] Fix reordering

Affects morx/kerx run-time only currently.  Will adjust their sanitize next.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11400

See comment.

Supercedes https://github.com/harfbuzz/harfbuzz/pull/1401
Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11366

This concludes null-size enforcement changes

See comments.

Since min_size is 0, Null() still accepts this type.

Cross-kerning can only take care of positioning vertically.  It doesn't
adjust mark advance...

Such that for Indic-like scripts (eg. Khmer), we don't do any fallback mark
advance-zeroing / positioning, but we do for Latin, etc.  Reuses preferences
of our script-specific OpenType shapers for those.

Fixes regression: https://github.com/harfbuzz/harfbuzz/issues/1393
Which means, fixes again: https://github.com/harfbuzz/harfbuzz/issues/1264
While not regressing: https://github.com/harfbuzz/harfbuzz/issues/1357

Needed for morx+kern mark-zeroing interaction.  All other scripts
work this way.

Over time it has become the same as default shaper.  So, remove.

../../src/hb-null.hh:53:39: warning: enum constant in boolean context [-Wint-in-bool-context]

This makes more of the gcc spurious warning:

../../src/hb-null.hh:53:39: warning: enum constant in boolean context [-Wint-in-bool-context]

But not going to let that defeat correct code.  Type to switch to clang
as my main compiler...

Fixes https://github.com/harfbuzz/harfbuzz/issues/1300

Not sure I've marked all such structs.  To be done as we discover.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1300

So, our fallback static_assert cannot be had more than once per line
of source.

Bots not happy with using "this" inside assertion...

This reverts 26566448