### 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)

ERR_FAIL_COND_V(!is_valid_terrain_peering_bit(p_peering_bit), -1);
	return terrain_peering_bits[p_peering_bit];
}
bool TileData::is_valid_terrain_peering_bit(TileSet::CellNeighbor p_peering_bit) const {
	ERR_FAIL_NULL_V(tile_set, false);
	return tile_set->is_valid_terrain_peering_bit(terrain_set, p_peering_bit);
}
TileSet::TerrainsPattern TileData::get_terrains_pattern() const {
	ERR_FAIL_NULL_V(tile_set, TileSet::TerrainsPattern());
	TileSet::TerrainsPattern output(tile_set, terrain_set);
	output.set_terrain(terrain);
	for (int i = 0; i < TileSet::CELL_NEIGHBOR_MAX; i++) {
		if (tile_set->is_valid_terrain_peering_bit(terrain_set, TileSet::CellNeighbor(i))) {
			output.set_terrain_peering_bit(TileSet::CellNeighbor(i), get_terrain_peering_bit(TileSet::CellNeighbor(i)));
		}
	}
	return output;
}
void TileData::set_navigation_polygon(int p_layer_id, Ref<NavigationPolygon> p_navigation_polygon) {
	ERR_FAIL_INDEX(p_layer_id, navigation.size());
	navigation.write[p_layer_id] = p_navigation_polygon;
	emit_signal(SNAME("changed"));
}
Ref<NavigationPolygon> TileData::get_navigation_polygon(int p_layer_id) const {
	ERR_FAIL_INDEX_V(p_layer_id, navigation.size(), Ref<NavigationPolygon>());
	return navigation[p_layer_id];
}
void TileData::set_probability(float p_probability) {