Add layer transforms, interaction in layer (#3906)

⚠️ Removes `Context::translate_layer`, replacing it with a sticky
`set_transform_layer`

Adds the capability to scale layers.
Allows interaction with scaled and transformed widgets inside
transformed layers.

I've also added a demo of how to have zooming and panning in a window
(see the video below).

This probably closes #1811. Having a panning and zooming container would
just be creating a new
`Area` with a new id, and applying zooming and panning with
`ctx.transform_layer`.

I've run the github workflow scripts in my repository, so hopefully the
formatting and `cargo cranky` is satisfied.

I'm not sure if all call sites where transforms would be relevant have
been handled. This might also be missing are transforming clipping
rects, but I'm not sure where / how to accomplish that. In the demo, the
clipping rect is transformed to match, which seems to work.


https://github.com/emilk/egui/assets/70821802/77e7e743-cdfe-402f-86e3-7744b3ee7b0f

---------

Co-authored-by: tweoss <fchua@puffer5.stanford.edu>
Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/epaint/src/mesh.rs

@@ -278,6 +278,13 @@ impl Mesh {
         }
     }
 
+    /// Transform the mesh in-place with the given transform.
+    pub fn transform(&mut self, transform: TSTransform) {
+        for v in &mut self.vertices {
+            v.pos = transform * v.pos;
+        }
+    }
+
     /// Rotate by some angle about an origin, in-place.
     ///
     /// Origin is a position in screen space.