This is for a test of the beta release patching process documented at https://github.com/flutter/flutter/wiki/Release-process

This is an hopefully harmless arbitrary commit chosen from those that landed after the last beta's commit.

* Extend Minikin to record the cluster identifier corresponding to each glyph
* Use the cluster values to determine the range of input code units that map
  to a glyph
* Use Minikin's libraries to find boundaries between graphemes within a
  ligature

Fixes https://github.com/flutter/flutter/issues/16151

libtxt: extend Minikin to find a fallback font if its font collection can not match a given character (#4623)

Minikin layout uses a FontCollection containing a list of pre-selected fonts
for a particular font family.  This patch extends the FontCollection to invoke
a hook provided by libtxt if layout sees a character that can not be rendered
by any font in the collection.

* Change the Minikin lock to a recursive mutex.  This is required because the
  fallback font provider may create new fonts during a layout operation that
  already holds the lock.
* Implement a fallback font provider hook that queries Skia for fonts matching
  an unrecognized character.
* Maintain a cache of fallback fonts.  Prepopulate the cache with fonts
  covering some commonly used character classes.
* Add a last resort font list for cases where Skia's font manager can not
  find any font for a character (similar to Blink's FontCache::getLastResortFallbackFont)

Revert "libtxt: extend Minikin to find a fallback font if its font collection can not match a given character (#4595)" (#4609)

This reverts commit 9a402241.

libtxt: extend Minikin to find a fallback font if its font collection can not match a given character (#4595)

Minikin layout uses a FontCollection containing a list of pre-selected fonts
for a particular font family.  This patch extends the FontCollection to invoke
a hook provided by libtxt if layout sees a character that can not be rendered
by any font in the collection.

* Change the Minikin lock to a recursive mutex.  This is required because the
  fallback font provider may create new fonts during a layout operation that
  already holds the lock.
* Implement a fallback font provider hook that queries Skia for fonts matching
  an unrecognized character.
* Maintain a cache of fallback fonts.  Prepopulate the cache with fonts
  covering some commonly used character classes.
* Add a last resort font list for cases where Skia's font manager can not
  find any font for a character (similar to Blink's FontCache::getLastResortFallbackFont)

The libtxt Paragraph class now runs the bidi algorithm itself, and each text
run passed to doLayout will be entirely LTR or RTL

* Added Windows support for libtxt in order to build flutter_tester.exe.

Relatively minor changes were needed to get libtxt building on Windows
(missing/incompatiable headers, the odd syscall, path separators, etc.).
Windows doesn't render text in the same way as other platforms, so some tests
that checked for specific pixel offsets are disabled.

Clang format the imported minikin sources, tests and benchmarks to match Flutter engine stylf. (#4018)