### 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)

r_ret = get_constant_linear_velocity(layer_index);
					return true;
				} else if (components[1] == "angular_velocity") {
					r_ret = get_constant_angular_velocity(layer_index);
					return true;
				} else if (components[1] == "polygons_count") {
					r_ret = get_collision_polygons_count(layer_index);
					return true;
				}
			} else if (components.size() == 3 && components[1].begins_with("polygon_") && components[1].trim_prefix("polygon_").is_valid_int()) {
				int polygon_index = components[1].trim_prefix("polygon_").to_int();
				ERR_FAIL_COND_V(polygon_index < 0, false);
				if (polygon_index >= physics[layer_index].polygons.size()) {
					return false;
				}
				if (components[2] == "points") {
					r_ret = get_collision_polygon_points(layer_index, polygon_index);
					return true;
				} else if (components[2] == "one_way") {
					r_ret = is_collision_polygon_one_way(layer_index, polygon_index);
					return true;
				} else if (components[2] == "one_way_margin") {
					r_ret = get_collision_polygon_one_way_margin(layer_index, polygon_index);
					return true;
				}
			}
		} else if (components.size() == 2 && components[0] == "terrains_peering_bit") {
			
			for (int i = 0; i < TileSet::CELL_NEIGHBOR_MAX; i++) {
				if (components[1] == TileSet::CELL_NEIGHBOR_ENUM_TO_TEXT[i]) {