Fix parsing of sequential key presses

[devbridie]

Currently, the implementation is not able to differentiate between a key release and a press following the same frame afterwards and being pressed for the entire duration of two presses. This is because Synthesia does not deactivate the pressed key light for that small interval.

I would propose a solution which looks at the black bar approaching the keyboard. Unfortunately, this is not ideal because a prediction would have to be made as to when that black bar approaches the keyboard. A variation in scrolling speed of the notes could lead to wrong predictions.