layout: Scale images in image_set by their specified resolution (servo#36374)

This PR makes it so the `resolution` factor in `image-set` also affects
the image size.
For instance, in the example below:

```css
background-image: image-set("./small.png" 1x, "./large.png" 2x);
```

if `large.png` is used, an image which is 32x32 will be rendered as
16x16. This is specified
in <https://drafts.csswg.org/css-images-4/#image-set-notation>.

Testing:
 - `css/css-images/image-set/image-set-resolution-002.html`

---------

Signed-off-by: tobinio <[email protected]>
Signed-off-by: Martin Robinson <[email protected]>
Co-authored-by: Martin Robinson <[email protected]>
Co-authored-by: Josh Matthews <[email protected]>

Author: 3 people

components/layout_2020/context.rs

@@ -5,6 +5,7 @@
 use std::sync::Arc;
 
 use base::id::PipelineId;
+use euclid::Size2D;
 use fnv::FnvHashMap;
 use fonts::FontContext;
 use fxhash::FxHashMap;
@@ -148,8 +149,7 @@ impl LayoutContext<'_> {
             Some(ImageOrMetadataAvailable::ImageAvailable { image, .. }) => {
                 self.handle_animated_image(node, image.clone());
                 let image_info = WebRenderImageInfo {
-                    width: image.width,
-                    height: image.height,
+                    size: Size2D::new(image.width, image.height),
                     key: image.id,
                 };
                 if image_info.key.is_none() {
@@ -188,10 +188,29 @@ impl LayoutContext<'_> {
                 )?;
                 Some(ResolvedImage::Image(webrender_info))
             },
-            Image::ImageSet(image_set) => image_set
-                .items
-                .get(image_set.selected_index)
-                .and_then(|image| self.resolve_image(node, &image.image)),
+            Image::ImageSet(image_set) => {
+                image_set
+                    .items
+                    .get(image_set.selected_index)
+                    .and_then(|image| {
+                        self.resolve_image(node, &image.image)
+                            .map(|info| match info {
+                                ResolvedImage::Image(mut image_info) => {
+                                    // From <https://drafts.csswg.org/css-images-4/#image-set-notation>:
+                                    // > A <resolution> (optional). This is used to help the UA decide
+                                    // > which <image-set-option> to choose. If the image reference is
+                                    // > for a raster image, it also specifies the image’s natural
+                                    // > resolution, overriding any other source of data that might
+                                    // > supply a natural resolution.
+                                    image_info.size = (image_info.size.to_f32() /
+                                        image.resolution.dppx())
+                                    .to_u32();
+                                    ResolvedImage::Image(image_info)
+                                },
+                                _ => info,
+                            })
+                    })
+            },
         }
     }
 }

components/layout_2020/display_list/mod.rs

@@ -61,8 +61,7 @@ pub use stacking_context::*;
 
 #[derive(Clone, Copy)]
 pub struct WebRenderImageInfo {
-    pub width: u32,
-    pub height: u32,
+    pub size: Size2D<u32, UnknownUnit>,
     pub key: Option<wr::ImageKey>,
 }
 
@@ -817,8 +816,8 @@ impl<'a> BuilderForBoxFragment<'a> {
                     // FIXME: https://drafts.csswg.org/css-images-4/#the-image-resolution
                     let dppx = 1.0;
                     let intrinsic = NaturalSizes::from_width_and_height(
-                        image_info.width as f32 / dppx,
-                        image_info.height as f32 / dppx,
+                        image_info.size.width as f32 / dppx,
+                        image_info.size.height as f32 / dppx,
                     );
                     let Some(image_key) = image_info.key else {
                         continue;
@@ -1007,8 +1006,8 @@ impl<'a> BuilderForBoxFragment<'a> {
                     return false;
                 };
 
-                width = image_info.width as f32;
-                height = image_info.height as f32;
+                width = image_info.size.width as f32;
+                height = image_info.size.height as f32;
                 NinePatchBorderSource::Image(key, ImageRendering::Auto)
             },
             Some(ResolvedImage::Gradient(gradient)) => {