Fix mouseup/mousedown event handler, not working properly with different mouse buttons

[noobiept]

Fixing this issue: #551

By setting the id with the mouse button, it will end up with a separate object for each button. Before the same object would be reused regardless of what mouse button was pressed.

Test I used:

// ...
circle.addEventListener('pressup', function(event)
    {
    console.log(event.type, event.nativeEvent.which);
    });
circle.addEventListener('mousedown', function(event)
    {
    console.log(event.type, event.nativeEvent.which);
    });
stage.addEventListener('stagemouseup', function(event)
    {
    console.log(event.type, event.nativeEvent.which);
    });
// ...

Press the mouse button and the middle mouse button at same time, then release them.
Before this patch you get:

mousedown 1
mousedown 2
stagemouseup 2
pressup 2

After:

mousedown 1
mousedown 2
stagemouseup 2
pressup 2
stagemouseup 1
pressup 1

[noobiept]

New test:

circle.addEventListener('pressup', function(event)
    {
    console.log(event.type, event.nativeEvent.which, event.pointerID);
    });
// etc

Result after patch:

mousedown 1 -1
mousedown 2 -2
stagemouseup 2 -2
pressup 2 -2
pressmove 1 -1
stagemouseup 1 -1
pressup 1 -1