Multiple priority managers (especially copper)

When using dual-playfield mode there are seldom enough colors to look good so making the copper manager merge multiple priorities of color changes (and other stuff) that don't line up with the same screen offset would be a welcome change from MrgCop()'s shortcomings.

An example of how it should work is that if both playfields contain tilemaps and need the vertical wrap-around modulo fix, the y offset may not come in the same order every time with both playfields scrolling at a different speed.

Complications that can emerge would be that occasionally both moves might attempt to happen at the same time in the list and one has to be delayed until the other one's move has been executed. This may give way to another issue of whether floods of moves need to be clipped or wrap to the next line.

Please discuss this with me and I can see if I can help if time permits.