### Godot version

5ee983188de97ae027f9b9c1443438063f708a7e

### System information

Windows 11

### Issue description

Godot editor will throw an error and fail to generate preview icons for terrain tiles which use a base image which is not RGBA8 format.
`core\io\image.cpp:2788 - Condition "format != p_src->format" is true.`

### Steps to reproduce

- Import a non-RGBA8 image into Godot.
- Create a TileMap
- Create an associated TileSet
- Create a terrain for this tileset
- Add the image as an atlas
- Go to the paint tab
- Paint the terrain on the TileSet
- An error will occur any time terrains are edited in the TileSet editor or selected in the TileMap editor terrain tab.

### Minimal reproduction project

[tileset_terrain_bug.zip](https://github.com/godotengine/godot/files/13264213/tileset_terrain_bug.zip)

}
					p.pop_front();
				}
			} else if (what == "priority_map") {
				Array p = p_value;
				Vector3 val;
				Vector2 v;
				int priority;
				while (p.size() > 0) {
					val = p[0];
					if (val.z > 1) {
						v.x = val.x;
						v.y = val.y;
						priority = (int)val.z;
						ctd->autotile_priority_map.insert(v, priority);
					}
					p.pop_front();
				}
			} else if (what == "z_index_map") {
				Array p = p_value;
				Vector3 val;
				Vector2 v;
				int z_index;
				while (p.size() > 0) {
					val = p[0];
					if (val.z != 0) {
						v.x = val.x;
						v.y = val.y;
						z_index = (int)val.z;
						ctd->autotile_z_index_map.insert(v, z_index);