During multi-step text input composing, such as with Chinese, Japanese,
and Korean text input, the framework sends embedders cursor rect updates
in the form of two messages:
* TextInput.setMarkedTextRect: notifies the embedder the size and
position of the composing text rect (or cursor when not composing) in
local coordinates.
* TextInput.setEditableSizeAndTransform: notifies the embedder of the
size of the EditableText and 4x4 transform matrix from local to
PipelineOwner.rootNode coordinates.
On receipt of either message, we cache a local copy on the
TextInputPlugin and notify the Win32FlutterWindow of the updated cursor
rect. In a followup patch, we update Win32FlutterWindow to implement the
Win32 input manager (IMM) calls required to position the IME candidates
window while editing.
