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>

examples/test_viewports/src/main.rs

@@ -428,7 +428,10 @@ fn drag_source<R>(
 
         if let Some(pointer_pos) = ui.ctx().pointer_interact_pos() {
             let delta = pointer_pos - res.response.rect.center();
-            ui.ctx().translate_layer(layer_id, delta);
+            ui.ctx().set_transform_layer(
+                layer_id,
+                eframe::emath::TSTransform::from_translation(delta),
+            );
         }
 
         res