"Pointer Down" listener triggered when mouse button released.

[dcturner]

Hi!
Thanks for the great work so far.

I'm seeing unwanted "pointer down" listeners firing when I release the mouse button.
Steps to reproduce

• Open the simple_2d example (heart rating)
• Press and hold a heart
• move your cursor to a different heart and release

Note: This makes press-and-hold interactions impossible at the moment, as no "Pointer Up" listener is ever called.