platform/windows: add optional DwmFlush() call to improve Windows capture

Invoke DwmFlush() before acquiring the next frame to alleviate visual stutter
during mouse movement at the cost of constraining the capture rate to the host's
monitor refresh.

Disabled by default; enable via "dwmflush" boolean configuration parameter.

sunshine/config.cpp

@@ -226,9 +226,10 @@ video_t video {
     1,
     -1 }, // vt
 
-  {}, // encoder
-  {}, // adapter_name
-  {}, // output_name
+  {},   // encoder
+  {},   // adapter_name
+  {},   // output_name
+  false // dwmflush
 };
 
 audio_t audio {};
@@ -735,6 +736,7 @@ void apply_config(std::unordered_map<std::string, std::string> &&vars) {
   string_f(vars, "encoder", video.encoder);
   string_f(vars, "adapter_name", video.adapter_name);
   string_f(vars, "output_name", video.output_name);
+  bool_f(vars, "dwmflush", video.dwmflush);
 
   path_f(vars, "pkey", nvhttp.pkey);
   path_f(vars, "cert", nvhttp.cert);