Add SKP occlusion test

tests/skp_occlusion.rs

@@ -0,0 +1,55 @@
+use lay_rs::{prelude::*, types::*};
+use skia_safe::{PictureRecorder, Rect};
+
+fn record_ops(with_bottom: bool) -> usize {
+    let engine = Engine::create(100.0, 100.0);
+    let parent = engine.new_layer();
+    parent.set_size(Size::points(100.0, 100.0), None);
+    engine.add_layer(&parent);
+
+    let top = engine.new_layer();
+    top.set_size(Size::points(100.0, 100.0), None);
+    top.set_background_color(
+        PaintColor::Solid {
+            color: Color::new_rgba255(0, 255, 0, 255),
+        },
+        None,
+    );
+    engine.append_layer(&top, parent.id);
+
+    if with_bottom {
+        let bottom = engine.new_layer();
+        bottom.set_size(Size::points(100.0, 100.0), None);
+        bottom.set_background_color(
+            PaintColor::Solid {
+                color: Color::new_rgba255(255, 0, 0, 255),
+            },
+            None,
+        );
+        engine.append_layer(&bottom, parent.id);
+    }
+
+    engine.update(0.016);
+
+    let mut recorder = PictureRecorder::new();
+    let canvas = recorder.begin_recording(Rect::from_wh(100.0, 100.0), None);
+    draw_scene(canvas, engine.scene(), engine.scene_root().unwrap());
+    let picture = recorder.finish_recording_as_picture(None).unwrap();
+    let path = std::env::temp_dir().join(if with_bottom {
+        "occlusion_with_bottom.skp"
+    } else {
+        "occlusion_top_only.skp"
+    });
+    std::fs::write(&path, picture.serialize().as_bytes()).unwrap();
+    picture.approximate_op_count()
+}
+
+#[test]
+fn occluded_layer_increases_layer_count() {
+    let without_bottom = record_ops(false);
+    let with_bottom = record_ops(true);
+    // Without the covering layer we record two layers: the parent and the top layer.
+    assert_eq!(without_bottom, 2);
+    // Adding a bottom layer that covers the others should still result in three recorded layers.
+    assert_eq!(with_bottom, 3);
+}