core, viewer: add support for WebP output

WebP can be selected by passing `--image-format webp` on the command
line. For typical Minecraft worlds, this results in a size reduction of
10-15% without increasing processing time.

src/core/tile_mipmapper.rs

@@ -144,7 +144,7 @@ where
 		}
 
 		image
-			.write_to(file, image::ImageFormat::Png)
+			.write_to(file, self.config.tile_image_format())
 			.context("Failed to save image")
 	}
 }