Improved texture loading (#3315)

* rework loading around `Arc<Loaders>`

* use `Bytes` instead of splitting api

* remove unwraps in `texture_handle`

* make `FileLoader` optional under `file` feature

* hide http load error stack trace from UI

* implement image fit

* support more image sources

* center spinner if we know size ahead of time

* allocate final size for spinner

* improve image format guessing

* remove `ui.image`, `Image`, add `RawImage`

* deprecate `RetainedImage`

* `image2` -> `image`

* add viewer example

* update `examples/image` + remove `svg` and `download_image` exapmles

* fix lints and tests

* fix doc link

* add image controls to `images` example

* add more `From` str-like types

* add api to forget all images

* fix max size

* do not scale original size unless necessary

* fix doc link

* add more docs for `Image` and `RawImage`

* make paint_at `pub`

* update `ImageButton` to use new `Image` API

* fix double rendering

* `SizeHint::Original` -> `Scale` + remove `Option` wrapper

* Update crates/egui/src/load.rs

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

* remove special `None` value for `forget`

* Update crates/egui/src/load.rs

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

* add more examples to `ui.image` + add `include_image` macro

* Update crates/egui/src/ui.rs

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

* update `menu_image_button` to use `ImageSource`

* `OrderedFloat::get` -> `into_inner`

* derive `Eq` on `SizedTexture`

* add `id` to loaders + `is_installed` check

* move `images` to demo + simplify `images` example

* log trace when installing loaders

* fix lint

* fix doc link

* add more documentation

* more `egui_extras::loaders` docs

* Update examples/images/src/main.rs

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

* update `images` example screenshots + readme

* remove unused `rfd` from `images` example

* Update crates/egui_extras/src/loaders/ehttp_loader.rs

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

* add `must_use` on `Image` and `RawImage`

* document `loaders::install` multiple call safety

* Update crates/egui_extras/Cargo.toml

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

* reshuffle `is_loader_installed`

* make `include_image` produce `ImageSource` + update docs

* update `include_image` docs

* remove `None` mentions from loader `forget`

* inline `From` texture id + size for `SizedTexture`

* add warning about statically known path

* change image load error + use in image button

* add `.size()` to `Image`

* Update crates/egui_demo_app/Cargo.toml

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

* add explanations to image viewer ui

---------

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

crates/egui_extras/src/loaders/svg_loader.rs

@@ -13,6 +13,10 @@ pub struct SvgLoader {
     cache: Mutex<HashMap<(String, SizeHint), Entry>>,
 }
 
+impl SvgLoader {
+    pub const ID: &str = egui::generate_loader_id!(SvgLoader);
+}
+
 fn is_supported(uri: &str) -> bool {
     let Some(ext) = Path::new(uri).extension().and_then(|ext| ext.to_str()) else { return false };
 
@@ -20,6 +24,10 @@ fn is_supported(uri: &str) -> bool {
 }
 
 impl ImageLoader for SvgLoader {
+    fn id(&self) -> &str {
+        Self::ID
+    }
+
     fn load(&self, ctx: &egui::Context, uri: &str, size_hint: SizeHint) -> ImageLoadResult {
         if !is_supported(uri) {
             return Err(LoadError::NotSupported);
@@ -39,7 +47,7 @@ impl ImageLoader for SvgLoader {
                 Ok(BytesPoll::Ready { bytes, .. }) => {
                     crate::log_trace!("started loading {uri:?}");
                     let fit_to = match size_hint {
-                        SizeHint::Original => usvg::FitTo::Original,
+                        SizeHint::Scale(factor) => usvg::FitTo::Zoom(factor.into_inner()),
                         SizeHint::Width(w) => usvg::FitTo::Width(w),
                         SizeHint::Height(h) => usvg::FitTo::Height(h),
                         SizeHint::Size(w, h) => usvg::FitTo::Size(w, h),
@@ -63,6 +71,10 @@ impl ImageLoader for SvgLoader {
         self.cache.lock().retain(|(u, _), _| u != uri);
     }
 
+    fn forget_all(&self) {
+        self.cache.lock().clear();
+    }
+
     fn byte_size(&self) -> usize {
         self.cache
             .lock()