Prior to this change, if none of the fonts specified in CSS contained a
glyph for a codepoint, we tried only one fallback font. If that font
didn't contain the glyph, we'd give up.

With this change, we try multiple fonts in turn. The font names we try
differ across each platform, and based on the codepoint we're trying to
match. The current implementation is heavily inspired by the analogous
code in Gecko, but I've used to ucd lib to make it more readable,
whereas Gecko matches raw unicode ranges.

This fixes some of the issues reported in #17267, although colour emoji
support is not implemented.

== Notes on changes to WPT metadata ==

=== css/css-text/i18n/css3-text-line-break-opclns-* ===

A bunch of these have started failing on macos when they previously
passed.

These tests check that the browser automatically inserts line breaks
near certain characters that are classified as "opening and closing
punctuation". The idea is that if we have e.g. an opening parenthesis,
it does not make sense for it to appear at the end of a line box; it
should "stick" to the next character and go into the next line box.

Before this change, a lot of these codepoints rendered as a missing
glyph on Mac and Linux. In some cases, that meant that the test was
passing.

After this change, a bunch of these codepoints are now rendering glyphs
on Mac (but not Linux). In some cases, the test should continue to pass
where it previously did when rendering with the missing glyph.

However, it seems this has also exposed a layout bug. The "ref" div in
these tests contains a <br> element, and it seems that this, combined
with these punctuation characters, makes the spacing between glyphs ever
so slightly different to the "test" div. (Speculation: might be
something to do with shaping?)

Therefore I've had to mark a bunch of these tests failing on mac.

=== css/css-text/i18n/css3-text-line-break-baspglwj-* ===

Some of these previously passed on Mac due to a missing glyph. Now that
we're rendering the correct glyph, they are failing.

=== css/css-text/word-break/word-break-normal-bo-000.html ===

The characters now render correctly on Mac, and the test is passing. But
we do not find a suitable fallback font on Linux, so it is still failing
on that platform.

=== css/css-text/word-break/word-break-break-all-007.html ===

This was previously passing on Mac, but only because missing character
glyphs were rendered. Now that a fallback font is able to be found, it
(correctly) fails.

=== mozilla/tests/css/font_fallback_* ===

These are new tests added in this commit. 01 and 02 are marked failing
on Linux because the builders don't have the appropriate fonts installed
(that will be a follow-up).

Fix build errors from rebase

FontTemplateDescriptor can no longer just derive(Hash). We need to
implement it on each component part, because the components now
generally wrap floats, which do not impl Hash because of NaN. However in
this case we know that we won't have a NaN, so it is safe to manually
impl Hash.

We can encounter control characters here, for example when processing a
<pre> element which contains newlines. Control characters are inherently
non-printing, therefore if we try to call find_by_codepoint for these
characters we will end up triggering an unnecessary font fallback
search.

FontTemplateData gets passed over IPC during the communication between
FontContext and FontCacheThread. Serializing and deserializing these
bytes is expensive, so this change ensures that we only do that when the
bytes can't be read from disk. A similar strategy is already used on
macos and windows.

The performance problem was particularly noticeable after implenting
font fallback, where the content process would potentially work through
a list of fonts, trying to find one which contains a certain glyph. That
could result in lots of font bytes going over IPC.

Before this change, if we needed to create a Font which we've already
created, but at a new size, then we'd fetch the FontTemplateInfo again.
If the bytes of the font are held in memory, then this could be
expensive as we need to pass those bytes over IPC.