mapgen set/point entry doesn't work with merged maps.

# Describe the bug

When trying to use
```
"set": [
{ "point": "trap", "id": "tr_rollmat", "x": [ 11, 15 ], "y": 9, "repeat": [ 2, 4 ] },
{ "point": "trap", "id": "tr_rollmat", "x": [ 56, 61 ], "y": 18, "repeat": [ 2, 4 ] },
{ "point": "trap", "id": "tr_rollmat", "x": [ 56, 61 ], "y": 19, "repeat": [ 2, 4 ] }
]
```
on a merged map (3x1), I discovered that set/point can't read past x: 23. In the above example, the 2nd two entries don't spawn any rollmats. The first entry will spawn a set of mats in each OMT (see screenshot)

# Steps To Reproduce
Try adding a set/point in a merged map that exists past the first OMT.

# Expected behavior

I'd like this to work like loot spawns for defining the x,y coordinates.

# Screenshots

No mats from the latter 2 entries in the exercise room. 3 instances of mats from the first entry in the hallway:

# Versions and configuration

- OS: Windows
- OS Version: 10.0 1903
- Game Version: 0.D-9180-gfcbe9b8 [64-bit]
- Graphics Version: Tiles
- Mods loaded: [
Dark Days Ahead [dda],
Disable NPC Needs [no_npc_food]
]

# Additional context

I can supply a save upon request but it has un-merged files in it that this example depends on.

Start by locating the mapgen handling for the "set" entries with "point" coordinates and trace how coordinates are interpreted across merged maps. Compare that behavior with loot spawn coordinate handling, then reproduce the 3x1 merged-map case; done means entries beyond the first OMT spawn at their specified coordinates.

Written by the indexing model from the issue text.