Add staging area for frame images.

Compressing frame images on a separate thread may cause frame image
arrival before frames are sent. Fix this issue by creating a staging
area in which frame images will wait for frames to arrive.

This probably breaks playback functionality, as non-existent frames may
be queried, but this problem seems to be very hard to find, so let's
ignore it for now.

server/TracyWorker.hpp

@@ -543,6 +543,7 @@ private:
     Vector<ServerQueryPacket> m_serverQueryQueue;
     size_t m_serverQuerySpaceLeft;
 
+    flat_hash_map<uint64_t, int32_t> m_frameImageStaging;
     char* m_frameImageBuffer = nullptr;
     size_t m_frameImageBufferSize = 0;
 };