Add egui::Scene for panning/zooming a Ui (#5505)

This is similar to `ScrollArea`, but:
* Supports zooming
* Has no scroll bars
* Has no limits on the scrolling

## TODO
* [x] Automatic sizing of `Scene`s outer bounds
* [x] Fix text selection in scenes
* [x] Implement `fit_rect`
* [x] Document / improve API

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/emath/src/ts_transform.rs

@@ -33,7 +33,7 @@ impl TSTransform {
 
     #[inline]
     /// Creates a new translation that first scales points around
-    /// `(0, 0)`, then translates them.  
+    /// `(0, 0)`, then translates them.
     pub fn new(translation: Vec2, scaling: f32) -> Self {
         Self {
             translation,
@@ -51,6 +51,11 @@ impl TSTransform {
         Self::new(Vec2::ZERO, scaling)
     }
 
+    /// Is this a valid, invertible transform?
+    pub fn is_valid(&self) -> bool {
+        self.scaling.is_finite() && self.translation.x.is_finite() && self.scaling != 0.0
+    }
+
     /// Inverts the transform.
     ///
     /// ```