### 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)

set_navigation_polygon(layer_index, polygon);
			return true;
		}
	} else if (components.size() == 2 && components[0] == "terrains_peering_bit") {
		
		for (int i = 0; i < TileSet::CELL_NEIGHBOR_MAX; i++) {
			TileSet::CellNeighbor bit = TileSet::CellNeighbor(i);
			if (components[1] == TileSet::CELL_NEIGHBOR_ENUM_TO_TEXT[i]) {
				set_terrain_peering_bit(bit, p_value);
				return true;
			}
		}
		return false;
	} else if (components.size() == 1 && components[0].begins_with("custom_data_") && components[0].trim_prefix("custom_data_").is_valid_int()) {
		
		int layer_index = components[0].trim_prefix("custom_data_").to_int();
		ERR_FAIL_COND_V(layer_index < 0, false);
		if (layer_index >= custom_data.size()) {
			if (tile_set) {
				return false;
			} else {
				custom_data.resize(layer_index + 1);
			}
		}
		set_custom_data_by_layer_id(layer_index, p_value);
		return true;
	}
	return false;
}
bool TileData::_get(const StringName &p_name, Variant &r_ret) const {