[host] use a local copy of the header and then update it all in one go

Writing to shared memory is much faster then reading as the shared
memory is not cached, this change ensures we are using a local copy
of the header performing the final update all in one go.

host/Service.h

@@ -54,7 +54,8 @@ private:
   HANDLE     m_timer;
   ICapture * m_capture;
 
-  KVMFRHeader * m_header;
+  KVMFRHeader   m_header;
+  KVMFRHeader * m_shmHeader;
   uint8_t     * m_frame[2];
   size_t        m_frameSize;
   uint64_t      m_dataOffset[2];