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

Thanks @jyviko.

crates/render/src/analysis.rs

@@ -0,0 +1,54 @@
+//! Surface analysis as a view draws it: the shading mode and the colours it uses.
+
+use solidcraft_geom::Vec3;
+
+/// Surface analysis colours, straight sRGB 0..1 as the shader takes them.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct AnalysisColors {
+    /// Draft Analysis: at least the draft angle, between, at most minus the angle.
+    pub draft: [[f32; 3]; 3],
+    /// Accessibility Analysis: reachable, blocked.
+    pub access: [[f32; 3]; 2],
+    /// Curvature Map: flat, the middle of the scale, at or below the analysis radius.
+    pub curvature: [[f32; 3]; 3],
+    /// Stripe the negative draft and blocked faces too, so colour is not the only cue.
+    pub hatch: bool,
+}
+
+impl AnalysisColors {
+    /// The light theme's colours (no colour vision mode): what headless pictures use.
+    pub const DEFAULT: AnalysisColors = AnalysisColors {
+        draft: [[0.20, 0.75, 0.30], [0.95, 0.80, 0.15], [0.85, 0.22, 0.20]],
+        access: [[0.20, 0.75, 0.30], [0.85, 0.22, 0.20]],
+        curvature: [[0.15, 0.35, 0.95], [0.10, 0.80, 0.25], [0.90, 0.15, 0.10]],
+        hatch: false,
+    };
+}
+
+impl Default for AnalysisColors {
+    fn default() -> Self {
+        AnalysisColors::DEFAULT
+    }
+}
+
+/// A way of shading the model's faces to judge their shape.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum AnalysisMode {
+    /// Reflected stripes: `stripes` across the view.
+    Zebra { stripes: f64 },
+    /// Faces by draft angle (degrees) to `pull`.
+    Draft { pull: Vec3, angle: f64 },
+    /// Colour by curvature, reaching the tight end at 1 / `radius`.
+    Curvature { radius: f64 },
+    /// Mirror finish reflecting a studio.
+    Environment,
+    /// Reachable along `dir` (facing it, nothing above) or blocked.
+    Access { dir: Vec3 },
+}
+
+/// A surface analysis shading a picture: the mode and its colours.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct SceneAnalysis {
+    pub mode: AnalysisMode,
+    pub colors: AnalysisColors,
+}