libtxt: track the font metrics of each run in order to calculate heights in GetRectsForRange (#4442)

Previously GetRectsForRange was assigning each rectangle a height matching the
height of the entire line.  If the line includes multiple text styles, callers
will expect each span's rectangle to reflect the height of that span.
(see text_painter_rtl_test.dart)

This fixes obfuscated snapshot mode.

This adds support for scrolling the primary scroll view to the top on
status bar touches, on the iPhone X.

Notes:
1. The iPhone X status bar doesn't change height when in in-call/etc.
   mode, and unlike other iPhones, does scroll to top when in in-call mode.
2. No matter which model of iOS device, the top safe area inset doesn't
   change when in in-call mode. In in-call mode, the OS reduces the app
   view height by 20px off the top, and the double-height 'in-call' status
   bar covers this new inset (outside the view) and there continues to be a
   20px safe area in the app.

On iOS 11, rather than comparing status bar height to a hardcoded 20px
'standard height' we now compare to the top safe area inset (which is
always the standard status bar height, regardless of device). On iOS
versions prior to iOS 11, we use the previous logic.

Fixes flutter/flutter#13439

Fixed failure on Windows in paragraph.cc which was surfaced by enabling flutter_tester.exe build on bots.

Keyboard height is now treated as a view inset, which could be used to
shrink the content area of the app as is done in the Material Scaffold,
rather than padding, which is used to indicate content areas where user
interaction should be avoided.

Fixes https://github.com/flutter/flutter/issues/13398

* a11y cursor movement by character

* focus fixes

* doc fixes

This allows for us to start migrating framework classes (notably
Scaffold) to use window.viewInsets.bottom to get the keyboard height.
Once framework updates have landed, we will land an engine patch to
expose the iOS bottom safe area inset as padding.

This change is already landed for Android.

Related issue: flutter/flutter#12098

* Call the ICU bidi API to divide the text into LTR/RTL runs and merge
  them with the styled runs defined by the application
* Maintain a list of glyph positions sorted by the order of the corresponding
  code units in the text, as well as a list of glyphs sorted by x/y coordinates
  in the layout

...by making it know about ALL the deprecated language and region codes.

Previously, we failed to clear/update the mark text range on updates
from the framework. This resulted in a crash if the incoming text value
was shorter than the current mark range when iOS attempts to extract the
mark range substring from the text.

Fixes flutter/flutter#12585

Fixes flutter/flutter#13308

Overlooked this case when I previously removed linear blending. MakeS32 creates a surface with a color space attached (triggering the linear blending behavior). MakeN32 creates a legacy surface (no color space), which matches what the other backend bindings do (including the GL surface factory, and the raster cache).

* Use the VM-specific wrapper for the kernel front-end to support VM-specific kernel transformations.
* Also, pass a flag to distinguish AOT mode from JIT mode to enable whole-program optimizations.

* Add synchronization to ResourceContext usage

* debug

* Use unique ptr factory

* formatting

* left more notes for future readers

* Roll dart to f5fc335a4cdfcf96ae550044c64044ffe599e595

* Update license hash

* Roll a little further to pick up a revert

* Update hash

Made changes to assets/ build/ flow/ runtime/ and shell/ to allow for
compilation on Windows.

* 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.

This drops the ViewInsets class in favour of the existing WindowPadding
class for representing the view insets.

* Roll dart version in flutter engine.

* Update license file.

Window.viewInsets is the set of window-relative insets that describe the
area of the window that an application may want to treat as effectively
reducing the size of the content. Typically this is due to system UI
that fully obscures underlying content, such as the keyboard.

This area differs from padding in that padding is the set of insets that
describe the area of the window that may be partially (or fully)
obscured by system UI or physical intrusions into the view area (e.g.
iPhone X sensor housing, status bar, or the iPhone X home indicator
widget).

This patch does not yet enable the iOS bottom edge safe area. Once the
framework has been updated to use viewInsets for bottom-edge occlusions
(today, the keyboard), the bottom safe area will be enabled and
framework patches that depend on it, landed.

* Removed stray character.

* Added missing include.

* Fixed formatting.