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/emath/src/lib.rs

@@ -36,6 +36,7 @@ mod rect;
 mod rect_transform;
 mod rot2;
 pub mod smart_aim;
+mod ts_transform;
 mod vec2;
 mod vec2b;
 
@@ -48,6 +49,7 @@ pub use {
     rect::*,
     rect_transform::*,
     rot2::*,
+    ts_transform::*,
     vec2::*,
     vec2b::*,
 };