Scroll at a consistent speed.

[mesilliac]

Fixes #3607

The controller_base::handle_scroll function is being called inside a loop that doesn't seem to be limited by anything. So if processing is fast, it loops and scrolls a lot, and if processing is slow, it doesn't.

I made it scroll by an amount proportional to the amount of time between calls to handle_scroll.

[stevecotton]

Looks and feels good to me. I'm not sure if the acceleration is necessary (as it seems to be full speed within about 3 frames), but the speed limit definitely is.

[CelticMinstrel]

I vote we merge this immediately.

[mesilliac]

I'm not sure if the acceleration is necessary

The slow start is mostly so all the variables (scrolling_, last_scroll_tick_ etc) get set correctly. It was simpler than writing extra handling code. The small bit of acceleration is a nice side-benefit.

[Vultraz]

I like this. :) Is it possible we could use a proper Bezier function to calculate position? ease-in-out maybe?

[mesilliac]

Yeah, applying an ease-in to scroll_amount should work. It would just need another variable to track the time since scroll start and apply the easing function based on that. I wasn't sure if there was an existing easing function / system to use so i didn't try anything fancy.

[Vultraz]

What about replacing last_scroll_tick_ with a scroll_start_ var. We don't (oddly enough) have any built-in easing functions (they should be added), so for now a helper function in the file would do.

[mesilliac]

Nah it still needs it for the time delta. As there's no endpoint for mouse/keyboard scrolling, only a direction, the speed can be eased but not the absolute position.

In any case it's pushing the scope for this a bit, as it's basically just a bugfix as is.