Currently the embedding API does not set "buttons" to pointer events, resulting in buttons = 0, which blocks adding support to buttons.

Embedding API should pass the correct buttons to pointer events:

1. Pointer events by mouse should set buttons to the buttons pressed.
2. Pointer events by touch should set buttons to 1 (kTouchContact).
3. Pointer events by stylus should set buttons to 1 (kStylusContact) as well as the physical buttons pressed.
4. Buttons should be non-zero when and only when the pointer is down.

Blocking: #31935
More discussion: #31884