[host] windows: detect whether screensaver is disabled in the guest

This will allow us to add an option to disable the screensaver on the client
when an application in the guest requests it. This behaviour may be useful
when the guest is doing media playback.

host/platform/Windows/CMakeLists.txt

@@ -33,6 +33,7 @@ target_link_libraries(platform_Windows
 	wtsapi32
 	psapi
 	shlwapi
+	powrprof
 )
 
 target_include_directories(platform_Windows

host/platform/Windows/src/platform.c

@@ -25,6 +25,8 @@ Place, Suite 330, Boston, MA 02111-1307 USA
 #include <shellapi.h>
 #include <shlwapi.h>
 #include <fcntl.h>
+#include <powrprof.h>
+#include <ntstatus.h>
 
 #include "interface/platform.h"
 #include "common/debug.h"
@@ -397,3 +399,24 @@ HWND os_getMessageWnd(void)
 {
   return app.messageWnd;
 }
+
+bool os_blockScreensaver()
+{
+  static bool      lastResult = false;
+  static ULONGLONG lastCheck  = 0;
+
+  ULONGLONG now = GetTickCount64();
+  if (now - lastCheck >= 1000)
+  {
+    ULONG executionState;
+    NTSTATUS status = CallNtPowerInformation(SystemExecutionState, NULL, 0,
+      &executionState, sizeof executionState);
+
+    if (status == STATUS_SUCCESS)
+      lastResult = executionState & ES_DISPLAY_REQUIRED;
+    else
+      DEBUG_ERROR("Failed to call CallNtPowerInformation(SystemExecutionState): %ld", status);
+    lastCheck = now;
+  }
+  return lastResult;
+}