Piano: Add piano roll

m_roll_notes[] is an array of 2 bools, pressed and playing. A roll note
is highlighted if it is pressed or in the currently playing column, but
the note is only actually played if the roll note is both pressed and
playing.

We change columns every m_tick which is currently 10 frames. The delay
is synchronised with this. change_roll_column() tracks the previous
column and current column to be played. Each roll note in the previous
column is set to "not playing" and the underlying audio note is turned
off if it was pressed. For the current column, each roll note is set to
playing and the underlying audio note is turned on if pressed.

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