decoder: Update dimension with frame info

In theory the header information doesn't has to be correct. There are no
known formats where this is the case, but just to be safe.

Closes #305

src/decoder/formats/glycin_proxy.rs

@@ -70,6 +70,10 @@ impl Glycin {
             let mut metadata: Metadata = Metadata::default();
             metadata.set_frame_info(frame.details);
             updater.send(DecoderUpdate::Metadata(Box::new(metadata)));
+            tiles.set_original_dimensions((
+                frame.texture.width() as u32,
+                frame.texture.height() as u32,
+            ));
 
             if let Some(delay) = frame.delay {
                 updater.send(DecoderUpdate::Animated);