### 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)

if (terrain_set == p_terrain_set) {
		if (terrain == p_index) {
			terrain = -1;
		} else if (terrain > p_index) {
			terrain -= 1;
		}
		for (int i = 0; i < 16; i++) {
			if (terrain_peering_bits[i] == p_index) {
				terrain_peering_bits[i] = -1;
			} else if (terrain_peering_bits[i] > p_index) {
				terrain_peering_bits[i] -= 1;
			}
		}
	}
}
void TileData::add_navigation_layer(int p_to_pos) {
	if (p_to_pos < 0) {
		p_to_pos = navigation.size();
	}
	ERR_FAIL_INDEX(p_to_pos, navigation.size() + 1);
	navigation.insert(p_to_pos, Ref<NavigationPolygon>());
}
void TileData::move_navigation_layer(int p_from_index, int p_to_pos) {