Correct modifier keys from mouse events

[jessegreenberg]

KeystateTracker.correctModifierKeys helps correct the state of KeyStateTracker when modifier keys are released/pressed without the browser receiving an event. It currently fires for keyboard events but should also fire for mouse events that support shiftKey, altKey, ctrlKey and metaKey properties. Otherwise mouse + keyboard interaction could be buggy if the browser window loses focus while modifier keys are down.

[jessegreenberg]

Additional notes:

A new function that corrects modifier keys needs to be very fast because it will run every mouse event. Refactor as much as possible, probably creating a new function just for mouse events.