Expose display device name as UTF-8.

This makes interaction with libOVR simpler, as it provides the display
device name in the same encoding.

src/win32_platform.h

@@ -205,6 +205,7 @@ typedef struct _GLFWmonitorWin32
     // This size matches the static size of DISPLAY_DEVICE.DeviceName
     WCHAR               adapterName[32];
     WCHAR               displayName[32];
+    char                nativeName[64];
     GLboolean           modeChanged;
 
 } _GLFWmonitorWin32;