Control wires should get pushed when they overlap a wide gate

[Strilanc]

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Currently, a controlled-not whose control wire crosses over a 2x2 density matrix will not cause a spacer column to be inserted. The result is the wire being in the way of the density matrix display, and possible confusion over whether or not it is conditioning the display.

Extract the logic used to draw the control wires into functions, and re-use those functions to cause spacers to be introduced in this situation.