Adjustable touchpad scroll speed

Thanks for working on egui, it is very impressive!

**Is your feature request related to a problem? Please describe.**

Scrolling with my trackpad on Wayland is very slow and I see no way to change it.

**Describe the solution you'd like**

An option in `Context::options` to set a factor which `smooth_scoll_delta` is multiplied by in `scroll_area.rs`.

**Describe alternatives you've considered**

`line_scroll_speed` doesn't affect touchpads, only mouse wheels.

**Additional context**

Start in scroll_area.rs and inspect Context::options to trace where smooth_scoll_delta is handled. Add the requested configurable factor for touchpad scrolling, while preserving the existing line_scroll_speed behavior for mouse wheels. Done means users can adjust touchpad scroll speed through Context::options on Wayland.

Written by the indexing model from the issue text.