Allow disabling animations on a ScrollArea (#4309)

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/master/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to add commits to your PR.
* Remember to run `cargo fmt` and `cargo cranky`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->

Adds a flag to ScrollArea that disables animations for the scroll_to_*
functions

crates/egui/src/containers/scroll_area.rs

@@ -176,6 +176,9 @@ pub struct ScrollArea {
     /// end position until user manually changes position. It will become true
     /// again once scroll handle makes contact with end.
     stick_to_end: Vec2b,
+
+    /// If false, `scroll_to_*` functions will not be animated
+    animated: bool,
 }
 
 impl ScrollArea {
@@ -219,6 +222,7 @@ impl ScrollArea {
             scrolling_enabled: true,
             drag_to_scroll: true,
             stick_to_end: Vec2b::FALSE,
+            animated: true,
         }
     }
 
@@ -393,6 +397,15 @@ impl ScrollArea {
         self
     }
 
+    /// Should the scroll area animate `scroll_to_*` functions?
+    ///
+    /// Default: `true`.
+    #[inline]
+    pub fn animated(mut self, animated: bool) -> Self {
+        self.animated = animated;
+        self
+    }
+
     /// Is any scrolling enabled?
     pub(crate) fn is_any_scroll_enabled(&self) -> bool {
         self.scroll_enabled[0] || self.scroll_enabled[1]
@@ -459,6 +472,7 @@ struct Prepared {
 
     scrolling_enabled: bool,
     stick_to_end: Vec2b,
+    animated: bool,
 }
 
 impl ScrollArea {
@@ -475,6 +489,7 @@ impl ScrollArea {
             scrolling_enabled,
             drag_to_scroll,
             stick_to_end,
+            animated,
         } = self;
 
         let ctx = ui.ctx().clone();
@@ -650,6 +665,7 @@ impl ScrollArea {
             viewport,
             scrolling_enabled,
             stick_to_end,
+            animated,
         }
     }
 
@@ -761,6 +777,7 @@ impl Prepared {
             viewport: _,
             scrolling_enabled,
             stick_to_end,
+            animated,
         } = self;
 
         let content_size = content_ui.min_size();
@@ -804,7 +821,9 @@ impl Prepared {
                     if delta != 0.0 {
                         let target_offset = state.offset[d] + delta;
 
-                        if let Some(animation) = &mut state.offset_target[d] {
+                        if !animated {
+                            state.offset[d] = target_offset;
+                        } else if let Some(animation) = &mut state.offset_target[d] {
                             // For instance: the user is continuously calling `ui.scroll_to_cursor`,
                             // so we don't want to reset the animation, but perhaps update the target:
                             animation.target_offset = target_offset;