Used by the routines at enter_arch_s, enter_arch_n, enter_arch_w and enter_arch_e.

The code expects a room's arches to be its first background objects, each two records (the two pillars, types 2 to 5), so the first pillars are records 4, 6, 8 and 10. The side an arch is on is recognised by the X+Y of its first pillar -- $51 north, $37 east, $C8 south, $AE west, from the arch data at arch_n on. The player takes that pillar's Z, which is what puts him at the right height in a raised arch; the head goes 12 above.

The search stops at the first record that is not an arch pillar; if no arch matches, Z is left alone.