### 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)

if (p_atlas_coords.x < 0 || p_atlas_coords.y < 0) {
		return false;
	}
	if (p_size.x <= 0 || p_size.y <= 0) {
		return false;
	}
	if (p_frames_count <= 0) {
		return false;
	}
	Size2i atlas_grid_size = get_atlas_grid_size();
	for (int frame = 0; frame < p_frames_count; frame++) {
		Vector2i frame_coords = p_atlas_coords + (p_size + p_animation_separation) * ((p_animation_columns > 0) ? Vector2i(frame % p_animation_columns, frame / p_animation_columns) : Vector2i(frame, 0));
		for (int x = 0; x < p_size.x; x++) {
			for (int y = 0; y < p_size.y; y++) {
				Vector2i coords = frame_coords + Vector2i(x, y);
				if (_coords_mapping_cache.has(coords) && _coords_mapping_cache[coords] != p_ignored_tile) {
					return false;
				}
				if (coords.x >= atlas_grid_size.x || coords.y >= atlas_grid_size.y) {
					return false;
				}
			}
		}
	}
	return true;
}
bool TileSetAtlasSource::has_tiles_outside_texture() const {