Headless pictures draw on the GPU, with the viewport's pipelines (CPU rasterizer as last tier) (#1555)

Thanks @jyviko.

crates/render/src/scene.rs

@@ -0,0 +1,65 @@
+//! What a picture of the model shows, independent of how it is drawn: meshes in colours, lines,
+//! the background, a section cut and a surface analysis. The GPU draws it with the viewport's
+//! pipelines (`solidcraft_gpu::capture`).
+
+use std::sync::Arc;
+
+use solidcraft_geom::{Mesh, Vec3};
+
+use crate::SceneAnalysis;
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct Rgb(pub u8, pub u8, pub u8);
+
+impl Rgb {
+    /// The colour as 0..1 per channel.
+    pub fn f(self) -> [f64; 3] {
+        [self.0 as f64 / 255.0, self.1 as f64 / 255.0, self.2 as f64 / 255.0]
+    }
+}
+
+pub struct SceneMesh {
+    pub mesh: Arc<Mesh>,
+    pub color: Rgb,
+    /// Faces in colours of their own: (B-rep face index, colour).
+    pub face_colors: Vec<(u32, Rgb)>,
+}
+
+pub struct SceneLine {
+    pub points: Vec<Vec3>,
+    pub color: Rgb,
+    /// Width in output pixels.
+    pub width: f64,
+    /// Draw over everything (sketch geometry being edited).
+    pub on_top: bool,
+}
+
+#[derive(Default)]
+pub struct Scene {
+    pub meshes: Vec<SceneMesh>,
+    pub lines: Vec<SceneLine>,
+    /// The ground grid: drawn under everything (the model hides it; it never hides the model).
+    pub grid: Vec<SceneLine>,
+    /// Background gradient top → bottom.
+    pub background: Option<(Rgb, Rgb)>,
+    /// Scene radius for clipping planes.
+    pub radius: f64,
+    /// A section cut: (normal, d); what lies where normal·p > d is cut away, and the inside
+    /// shows as a hatched cap.
+    pub clip: Option<(Vec3, f64)>,
+    /// The colour the section cap is tinted toward (default: [`SECTION_CAP`]).
+    pub cap: Option<Rgb>,
+    /// A surface analysis shading the meshes (zebra, draft, curvature map, ...).
+    pub analysis: Option<SceneAnalysis>,
+}
+
+/// The section cap's default tint, as in the viewport.
+pub const SECTION_CAP: Rgb = Rgb(214, 128, 72);
+
+/// RGBA pixels (`w × h`) to PNG bytes.
+pub fn png(w: usize, h: usize, rgba: Vec<u8>) -> Option<Vec<u8>> {
+    let img = image::RgbaImage::from_raw(u32::try_from(w).ok()?, u32::try_from(h).ok()?, rgba)?;
+    let mut buf = std::io::Cursor::new(Vec::new());
+    img.write_to(&mut buf, image::ImageFormat::Png).ok()?;
+    Some(buf.into_inner())
+}