returns true if Steam & the Steam Overlay are running in Big Picture mode
Games much be launched through the Steam client to enable the Big Picture overlay. During development,
a game can be added as a non-steam game to the developers library to test this feature
public static bool IsSteamInBigPictureMode() { InteropHelp.TestIfAvailableClient(); return NativeMethods.ISteamUtils_IsSteamInBigPictureMode(); }