In main branch, the Windows engine sends mouse leave event without setting mouse coords from initial zero value.
It causes that MouseRegion widget receives hover event to coords (0, 0), and exit event with coords (0, 0).

Demo:

import 'package:flutter/material.dart';

void main() {
  runApp(Container(
    color: Colors.red,
    child: MouseRegion(
      onHover: (event) {
        print("hover: ${event.toString()}");
      },
      onExit: (event) {
        print("exit: ${event.toString()}");
      },
    ),
  ));
}

This PR adds sending the coords:

• When touch input, window_win32 sends given coords.
• When mouse input, window_win32 stores coords when WM_MOUSEMOVE, and sends its coords when WM_MOUSELEAVE.

This PR will fix flutter/flutter#100213

No changes in flutter/tests repo.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.