Introduce global zoom_factor

[emilk]

• Closes Introduce a persistent zoom_factor #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.

[Barugon]

I really feel that this should not be enabled by default.