Win32: Fix screensaver and blanking prevention

On Vista and later, Handling WM_SYSCOMMAND is not enough to prevent
password protected screensavers or monitor blanking.

Fixes #851.

src/win32_platform.h

@@ -248,6 +248,7 @@ typedef struct _GLFWlibraryWin32
 {
     HWND                helperWindowHandle;
     DWORD               foregroundLockTimeout;
+    int                 acquiredMonitorCount;
     char*               clipboardString;
     char                keyName[64];
     short int           keycodes[512];