### 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)

Array p = p_value;
					Vector2i last_coord;
					while (p.size() > 0) {
						if (p[0].get_type() == Variant::VECTOR2) {
							last_coord = p[0];
						} else if (p[0].get_type() == Variant::INT) {
							ctd->autotile_bitmask_flags.insert(last_coord, p[0]);
						}
						p.pop_front();
					}
				}
			} else if (what == "occluder_map") {
				Array p = p_value;
				Vector2 last_coord;
				while (p.size() > 0) {
					if (p[0].get_type() == Variant::VECTOR2) {
						last_coord = p[0];
					} else if (p[0].get_type() == Variant::OBJECT) {
						ctd->autotile_occluder_map.insert(last_coord, p[0]);
					}
					p.pop_front();
				}
			} else if (what == "navpoly_map") {
				Array p = p_value;
				Vector2 last_coord;
				while (p.size() > 0) {
					if (p[0].get_type() == Variant::VECTOR2) {
						last_coord = p[0];
					} else if (p[0].get_type() == Variant::OBJECT) {
						ctd->autotile_navpoly_map.insert(last_coord, p[0]);