GUAC-935: Only update display size if it's been longer than 250ms.

Modification to the existing use of the Display Update channel, limiting the frequency of resizing to no more than once every 250ms, as otherwise Windows tends to permanently hang.
Definitely should, yes.

I want to point out that this particular one is a handler that's passed to FreeRDP, and its parameters are dictated by that. That doesn't help us here, obviously... since there are no FreeRDP API docs.