Add smooth scroll to the scroll view.

[t800-x]

Added a smooth_scroll style that animates the scroll ticks with a linear animation. Had to move the set_positions() call out of apply_scroll_delta() cause while animating the tree is being modified and read at the same time, and it was causing understory_box_tree to panic and crash.

I tried to use exponential decay instead of a linear curve but had problems chaining scroll ticks in quick succession. Did the best I could.

[jrmoulton]

Exciting! Can you instead let the style prop be a DirectTranstion instead of just a bool. that should help clean up the state management as well

[t800-x]

Alright mate, done that. Also fixed a clippy warning in tiny_skia/src/lib.rs