Number of screens in level (equivalent to the "number of screens" setting in Lunar Magic). Note that there is no differentiation here between horizontal and vertical levels, so you will need to check that seperately before using this address to handle the maximum X or Y position of something. See $5E instead for specifically horizontal levels, or $5F for specifically vertical levels.
Also set to FF in Ludwig and Reznor's boss rooms. Instead, those rooms used a fixed width of 1.5 screens.
