### 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)

int index = tile_data->get_collision_polygons_count(0) - 1;
								tile_data->set_collision_polygon_one_way(0, index, csd.one_way);
								tile_data->set_collision_polygon_one_way_margin(0, index, csd.one_way_margin);
								tile_data->set_collision_polygon_points(0, index, polygon);
							}
						}
					}
				}
				
				if (!compatibility_size_count.has(ctd->region.get_size())) {
					compatibility_size_count[ctd->region.get_size()] = 0;
				}
				compatibility_size_count[ctd->region.get_size()]++;
			} break;
			case COMPATIBILITY_TILE_MODE_AUTO_TILE: {
				
				WARN_PRINT_ONCE("Could not convert 3.x autotiles to 4.x. This operation cannot be done automatically, autotiles must be re-created using the terrain system.");
			} break;
			case COMPATIBILITY_TILE_MODE_ATLAS_TILE: {
				atlas_source->set_margins(ctd->region.get_position());
				atlas_source->set_separation(Vector2i(ctd->autotile_spacing, ctd->autotile_spacing));
				atlas_source->set_texture_region_size(ctd->autotile_tile_size);
				Size2i atlas_size = ctd->region.get_size() / (ctd->autotile_tile_size + atlas_source->get_separation());
				for (int i = 0; i < atlas_size.x; i++) {
					for (int j = 0; j < atlas_size.y; j++) {
						Vector2i coords = Vector2i(i, j);
						for (int flags = 0; flags < 8; flags++) {
							bool flip_h = flags & 1;
							bool flip_v = flags & 2;
							bool transpose = flags & 4;