Move smooth scroll timers to a separate thread, instead of using SetTimer

[goodtrailer]

Potentially addresses #4122

Note. To avoid confusion, I'm going to refer to middle-click scroll as "auto scroll." And I'm going to refer to smoothly animated/interpolated scrolling (enabled by the SmoothScroll=true config) as "interpolated scroll." In the code, "auto scroll" is referred to as SMOOTHSCROLL, while "interpolated scroll" is referred to as smoothScroll. This could be confusing, which is why I call them "auto scroll" and "interpolated scroll."

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Discussion #4122 complains that "auto scroll" is too scratchy, and I noticed that this is possibly because it uses SetTimer(). Also, "interpolated scroll" uses SetTimer() as well. An issue is that SetTimer() allows a minimum of 10ms/100fps (according to the docs, but it seems the true minimum is closer to 16ms/60fps in my testing). Many displays run at much higher than 100Hz (e.g. mine runs at 144Hz), so this results in a very scratchy appearance.

A relatively non-intrusive solution is to replace the timer with a thread (see ScrollTimerThread()) which sends WM_TIMER messages to the canvas window. Since we use our own thread instead of SetTimer(), we can run at the monitor's refresh rate, instead of a capped 10ms/100fps. Note that using this method, not very much code needs changing. We still use WM_TIMER messages, which can be handled in the exact same way as before. The only difference is that instead of SetTimer()/KillTimer(), we have to call SetEvent()/ResetEvent() on an Event object which the thread is waiting on. Also, we have to account for the delta time (Δt) so that scrolling isn't faster/slower depending on refresh rate (i.e. for "auto scroll," a 144Hz monitor and a 60Hz monitor should have the same scroll speed).

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This is a draft PR because I just wanted to show a proof-of-concept that improves scrolling on high-refresh displays, in case it is helpful at all. It's not perfect, since WM_TIMER messages are low-priority. But it's relatively simple and non-intrusive, which is why I wanted to bring it up.