FIX: reset decode buffer zero when scale width changed

Change-Id: Iaa2f99111dd5f7228b7b25e1be0a8cbdbfe982a6
Jira: STUDIO-8422
(cherry picked from commit 659ebc7d07a8f6045ba5443141b44277d7257cec)

src/slic3r/GUI/AVVideoDecoder.hpp

@@ -34,6 +34,7 @@ private:
     AVFrame *       frame_     = nullptr;
     SwsContext *    sws_ctx_   = nullptr;
     bool got_frame_ = false;
+    int width_ { 0 }; // scale result width
     std::vector<uint8_t> bits_;
 };