[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/src/app.c

@@ -206,6 +206,7 @@ static int frameThread(void * opaque)
     fi->pitch             = frame.pitch;
     fi->offset            = pageSize - FrameBufferStructSize;
     fi->mouseScalePercent = app.iface->getMouseScale();
+    fi->blockScreensaver  = os_blockScreensaver();
     frameValid            = true;
 
     // put the framebuffer on the border of the next page