Software renderer: fix drawing background of rotated screen

We need to draw the background over the rotated rectangle otherwise we
don't fill the whole background.

Regression from #7685

(also cleanup a useless blend that was leftover from the same change)

Adapt the screenshot test to have at least one non-square "window" so
that we catch the problem in the future

internal/core/software_renderer.rs

@@ -588,13 +588,11 @@ impl SoftwareRenderer {
                 };
                 drop(i);
 
-                let mut bg = PremultipliedRgbaColor::background();
-                // TODO: gradient background
-                PremultipliedRgbaColor::blend(&mut bg, background.color().into());
                 renderer.actual_renderer.processor.dirty_region = dirty_region.clone();
                 renderer.actual_renderer.processor.process_rectangle_impl(
-                    PhysicalRect::from_size(euclid::Size2D::new(size.width, size.height)),
-                    bg,
+                    screen_rect.transformed(rotation),
+                    // TODO: gradient background
+                    background.color().into(),
                     CompositionMode::Source,
                 );
 

tests/screenshots/build.rs

@@ -102,6 +102,17 @@ fn main() -> std::io::Result<()> {
         });
         let skip_clipping = source.contains("SKIP_CLIPPING");
 
+        let needle = "SIZE=";
+        let (size_w, size_h) = source.find(needle).map_or((64, 64), |p| {
+            source[p + needle.len()..]
+                .find(char::is_whitespace)
+                .and_then(|end| source[p + needle.len()..][..end].split_once('x'))
+                .and_then(|(w, h)| Some((w.parse().ok()?, h.parse().ok()?)))
+                .unwrap_or_else(|| {
+                    panic!("Cannot parse {needle} for {}", testcase.relative_path.display())
+                })
+        });
+
         let mut output = BufWriter::new(std::fs::File::create(
             Path::new(&std::env::var_os("OUT_DIR").unwrap()).join(format!("{module_name}.rs")),
         )?);
@@ -122,7 +133,7 @@ fn main() -> std::io::Result<()> {
     use crate::testing;
 
     let window = testing::init_swr();
-    window.set_size(slint::PhysicalSize::new(64, 64));
+    window.set_size(slint::PhysicalSize::new({size_w}, {size_h}));
     let screenshot = {reference_path};
     let options = testing::TestCaseOptions {{ rotation_threshold: {rotation_threshold}f32, skip_clipping: {skip_clipping} }};
 

tests/screenshots/cases/software/basic/translucent-background.slint

@@ -1,8 +1,10 @@
 // Copyright © SixtyFPS GmbH <info@slint.dev>
 // SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
 
+// SIZE=60x64
+
 export component TestCase inherits Window {
-    width: 64px;
+    width: 60px;
     height: 64px;
 
     background: #2360e62b;

tests/screenshots/testing.rs

@@ -75,7 +75,8 @@ pub fn screenshot(
             LogicalRect::from_size(euclid::size2(width as f32, height as f32)).into(),
         );
         renderer.set_rendering_rotation(rotated);
-        renderer.render(buffer.make_mut_slice(), width as usize);
+        let stride = buffer.width() as usize;
+        renderer.render(buffer.make_mut_slice(), stride);
         renderer.set_rendering_rotation(RenderingRotation::NoRotation);
     });
 
@@ -133,7 +134,11 @@ fn compare_images(
         let reference = image_buffer(reference_path)
             .map_err(|image_err| format!("error loading reference image: {image_err:#}"))?;
 
-        if reference.size() != screenshot.size() {
+        let mut ref_size = reference.size();
+        if matches!(rotated, RenderingRotation::Rotate90 | RenderingRotation::Rotate270) {
+            std::mem::swap(&mut ref_size.width, &mut ref_size.height);
+        }
+        if ref_size != screenshot.size() {
             return Err(format!(
                 "image sizes don't match. reference size {:#?} rendered size {:#?}",
                 reference.size(),