fix: ffmpeg http camera stream jittering due to incomplete frames

src/slic3r/GUI/AVVideoDecoder.hpp

@@ -36,6 +36,14 @@ public:
 
     bool toWxBitmap(wxBitmap &bitmap, wxSize const & size);
 
+    // Native size of the most recently decoded frame, or an unspecified size if
+    // nothing has decoded yet. Lets a caller learn the video dimensions when the
+    // container/probe could not report them up front.
+    wxSize decoded_frame_size() const
+    {
+        return got_frame_ && frame_ ? wxSize{frame_->width, frame_->height} : wxSize{};
+    }
+
 private:
     AVCodecContext *codec_ctx_ = nullptr;
     AVFrame *       frame_     = nullptr;