Fix break when the OS resizes our window in `set_mode`

[oddbookworm]

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Hopefully fixes #2991. I suspect that there's a lot I missed, so @robertpfeiffer @ankith26 @Starbuck5 please take a long look at these changes.

I haven't found a reliable way we can predict what the size the OS will force small windows to is, so I resorted to seeing what SDL says it is. Maybe I could just manually go through the event queue instead of pumping and grabbing the size from there?