video: fix screen sharing on macOS

Because media encoder flush() involves encoding we should change
initialized flag after calling flush(), otherwise, it could lead to
new stream initializing.

jami-client-macos#299

Change-Id: I88d5648a6f252bd78776f505f891f09a4e5b12e1

src/media/media_encoder.cpp

@@ -1340,13 +1340,12 @@ MediaEncoder::getScaledSWFrame(const VideoFrame& input)
 void
 MediaEncoder::resetStreams(int width, int height)
 {
-    // Only called by VideoSender!
-    initialized_ = false;
     videoOpts_.width = width;
     videoOpts_.height = height;
 
     try {
         flush();
+        initialized_ = false;
         if (outputCtx_) {
             for (auto encoderCtx : encoders_) {
                 if (encoderCtx) {