Add test case for third culled background

sparse_strips/vello_sparse_tests/snapshots/left_cull_triangle_expands_below_viewport.png

diff --git a/sparse_strips/vello_sparse_tests/snapshots/left_cull_triangle_expands_below_viewport.png b/sparse_strips/vello_sparse_tests/snapshots/left_cull_triangle_expands_below_viewport.png
new file mode 100644
index 0000000..c8a3313
--- /dev/null
+++ b/sparse_strips/vello_sparse_tests/snapshots/left_cull_triangle_expands_below_viewport.png
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:d149bb0e1b096bb5ebc86f2ec1a6f670e709421d293677c558f4bcc18bc113dd
+size 374

sparse_strips/vello_sparse_tests/tests/basic.rs

diff --git a/sparse_strips/vello_sparse_tests/tests/basic.rs b/sparse_strips/vello_sparse_tests/tests/basic.rs
index f689ad6..4d98383 100644
--- a/sparse_strips/vello_sparse_tests/tests/basic.rs
+++ b/sparse_strips/vello_sparse_tests/tests/basic.rs
@@ -674,6 +674,18 @@
     ctx.fill_rect(&rect_bot);
 }
 
+#[vello_test(width = 30, height = 60)]
+fn left_cull_triangle_expands_below_viewport(ctx: &mut impl Renderer) {
+    let mut path = BezPath::new();
+    path.move_to((15.0, 2.0));
+    path.line_to((52.0, 72.0));
+    path.line_to((-22.0, 72.0));
+    path.close_path();
+
+    ctx.set_paint(REBECCA_PURPLE);
+    ctx.fill_path(&path);
+}
+
 #[vello_test(width = 30, height = 30)]
 fn left_cull_encloses_viewport(ctx: &mut impl Renderer) {
     let rect = Rect::new(-50.0, -50.0, 80.0, 80.0);