Preserve mouse capture per pointer

[tryk016]

Capture is currently stored by mouse button, so a second touch overwrites the
first one. Use the pointer ID as well, and don't treat a press from another
pointer as a double-click.

Enable multitouch on the controller's input view, including when Metal replaces it.

Adds regression tests for simultaneous touches, release order, mouse buttons,
re-entry and widget deletion. Tested with TempestTests on Windows, Linux and
locally on macOS, plus two-finger gestures in the iOS 18 and 27 simulators.

[Try]

Whole std::map is too heavy container for this use-case. In most cases this container will have 0-2 entries. Something like flat_map would be the right tool in principle, but it requires C++23.

Something like this should fit better:

struct MouseCapture {
  Event::MouseButton         button = {};
  int                        id     = 0;
  std::weak_ptr<Widget::Ref> ref    = {};
  };
std::vector<MouseCapture> mouseUp;

[tryk016]

Replaced the map with a vector of MouseCapture, as suggested.