feat: add shadow coloring

src/fractal/renderer.rs

@@ -46,6 +46,7 @@ fn color_differs(a: &Uniforms, b: &Uniforms) -> bool {
     a.color_offset != b.color_offset
         || a.color_scale != b.color_scale
         || a.palette_id != b.palette_id
+        || a.shadow != b.shadow
 }
 
 /// GPU-side view + coloring parameters. Layout must match `Uniforms` in the
@@ -83,8 +84,8 @@ pub struct Uniforms {
     pub lambda_l: [f32; 2],
     /// 0 = escape-time coloring, 1 = distance-estimation shading.
     pub de_coloring: u32,
-    /// Padding to a 16-byte multiple (uniform buffer requirement).
-    pub _pad: [u32; 1],
+    // 0 = classic colors, 1 = shadows
+    pub shadow: u32,
 }
 
 /// Offscreen textures for the two-pass render, recreated whenever the widget's