### 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)

per_terrain_pattern_tiles[terrain_set][terrains_pattern].insert(cell);
							}
							
							for (int i = 0; i < TileSet::CELL_NEIGHBOR_MAX; i++) {
								CellNeighbor bit = CellNeighbor(i);
								if (is_valid_terrain_peering_bit(terrain_set, bit)) {
									int terrain = terrains_pattern.get_terrain_peering_bit(bit);
									if (terrain >= 0) {
										per_terrain_pattern_tiles[terrain_set][terrains_pattern].insert(cell);
									}
								}
							}
						}
					}
				}
			}
		}
		
		for (int i = 0; i < terrain_sets.size(); i++) {
			TileSet::TerrainsPattern empty_pattern(this, i);
			TileMapCell empty_cell;
			empty_cell.source_id = TileSet::INVALID_SOURCE;
			empty_cell.set_atlas_coords(TileSetSource::INVALID_ATLAS_COORDS);
			empty_cell.alternative_tile = TileSetSource::INVALID_TILE_ALTERNATIVE;
			per_terrain_pattern_tiles[i][empty_pattern].insert(empty_cell);
		}
		terrains_cache_dirty = false;
	}
}
void TileSet::_compute_next_source_id() {