Introduce global zoom_factor (#3608)

* Closes https://github.com/emilk/egui/issues/3602

You can now zoom any egui app by pressing Cmd+Plus, Cmd+Minus or Cmd+0,
just like in a browser. This will change the current `zoom_factor`
(default 1.0) which is persisted in the egui memory, and is the same for
all viewports.
You can turn off the keyboard shortcuts with `ctx.options_mut(|o|
o.zoom_with_keyboard = false);`

`zoom_factor` can also be explicitly read/written with
`ctx.zoom_factor()` and `ctx.set_zoom_factor()`.

This redefines `pixels_per_point` as `zoom_factor *
native_pixels_per_point`, where `native_pixels_per_point` is whatever is
the native scale factor for the monitor that the current viewport is in.

This adds some complexity to the interaction with winit, since we need
to know the current `zoom_factor` in a lot of places, because all egui
IO is done in ui points. I'm pretty sure this PR fixes a bunch of subtle
bugs though that used to be in this code.

`egui::gui_zoom::zoom_with_keyboard_shortcuts` is now gone, and is no
longer needed, as this is now the default behavior.

`Context::set_pixels_per_point` is still there, but it is recommended
you use `Context::set_zoom_factor` instead.

crates/egui_demo_app/src/main.rs

@@ -4,6 +4,22 @@
 
 // When compiling natively:
 fn main() -> Result<(), eframe::Error> {
+    for arg in std::env::args().skip(1) {
+        match arg.as_str() {
+            "--profile" => {
+                #[cfg(feature = "puffin")]
+                start_puffin_server();
+
+                #[cfg(not(feature = "puffin"))]
+                panic!("Unknown argument: {arg} - you need to enable the 'puffin' feature to use this.");
+            }
+
+            _ => {
+                panic!("Unknown argument: {arg}");
+            }
+        }
+    }
+
     {
         // Silence wgpu log spam (https://github.com/gfx-rs/wgpu/issues/3206)
         let mut rust_log = std::env::var("RUST_LOG").unwrap_or_else(|_| "info".to_owned());
@@ -33,3 +49,28 @@ fn main() -> Result<(), eframe::Error> {
         Box::new(|cc| Box::new(egui_demo_app::WrapApp::new(cc))),
     )
 }
+
+#[cfg(feature = "puffin")]
+fn start_puffin_server() {
+    puffin::set_scopes_on(true); // tell puffin to collect data
+
+    match puffin_http::Server::new("127.0.0.1:8585") {
+        Ok(puffin_server) => {
+            eprintln!("Run:  cargo install puffin_viewer && puffin_viewer --url 127.0.0.1:8585");
+
+            std::process::Command::new("puffin_viewer")
+                .arg("--url")
+                .arg("127.0.0.1:8585")
+                .spawn()
+                .ok();
+
+            // We can store the server if we want, but in this case we just want
+            // it to keep running. Dropping it closes the server, so let's not drop it!
+            #[allow(clippy::mem_forget)]
+            std::mem::forget(puffin_server);
+        }
+        Err(err) => {
+            eprintln!("Failed to start puffin server: {err}");
+        }
+    };
+}