Final polish for new image loading (#3328)

* add egui logo to widget gallery

* improve "no image loaders" error message

* rework static URIs to accept `Cow<'static>`

* remove `RetainedImage` from `http_app` in `egui_demo_app`

* hide `RetainedImage` from docs

* use `ui.image`/`Image` over `RawImage`

* remove last remanant of `RawImage`

* remove unused doc link

* add style option to disable image spinners

* use `Into<Image>` instead of `Into<ImageSource>` to allow configuring the underlying image

* propagate `image_options` through `ImageButton`

* calculate image size properly in `Button`

* properly calculate size in `ImageButton`

* Update crates/egui/src/widgets/image.rs

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* improve no image loaders error message

* add `size()` helper to `TexturePoll`

* try get size from poll in `Button`

* add `paint_at` to `Spinner`

* use `Spinner::paint_at` and hover on image button response

* `show_spinner` -> `show_loading_spinner`

* avoid `allocate_ui` in `Image` when painting spinner

* make icon smaller + remove old texture

* add `load_and_calculate_size` + expose `paint_image_at`

* update `egui_plot` to paint image in the right place

* Add helpers for painting an ImageSource directly

* Use max_size=INF as default

* Use new API in WidgetGallery

* Make egui_demo_app work by default

* Remove Option from scale

* Refactor ImageSize

* Fix docstring

* Small refactor

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui_glium/examples/native_texture.rs

@@ -30,8 +30,7 @@ fn main() {
                 egui::SidePanel::left("my_side_panel").show(egui_ctx, |ui| {
                     if ui
                         .add(egui::Button::image_and_text(
-                            texture_id,
-                            button_image_size,
+                            (texture_id, button_image_size),
                             "Quit",
                         ))
                         .clicked()