Video backends that run asynchronously can wind up in a race situation if a mode or position update request queues up a fullscreen enter request while an unprocessed asynchronous leave request is in flight, and the mode switch or position update request is processed after the leave request, causing the window to erroneously return to fullscreen.
Instead of the internal SDL_UpdateFullscreenMode and backend SetWindowFullscreen functions taking a boolean value, use an enum that allows the backends to distinguish between "this is an explicit fullscreen enter/leave request", and "this is an update request to change the mode or position". Communicating the specific intent allows the backend to early-out when required, which prevents windows from erroneously returning to fullscreen due to update requests made after a leave request, and allows for the removal of some internal synchronization previously needed to (attempt to) work around this, which improves overall performance while also increasing robustness.
This is only relevant to the internal functions, and nothing in the public-facing API has been changed.
0e1f3b681f