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