Add test case for OOB access in multi-threading

sparse_strips/vello_sparse_tests/tests/issues.rs

diff --git a/sparse_strips/vello_sparse_tests/tests/issues.rs b/sparse_strips/vello_sparse_tests/tests/issues.rs
index d6996cb..575b534 100644
--- a/sparse_strips/vello_sparse_tests/tests/issues.rs
+++ b/sparse_strips/vello_sparse_tests/tests/issues.rs
@@ -7,8 +7,10 @@
 use vello_common::color::palette::css::{DARK_BLUE, LIME, REBECCA_PURPLE};
 use vello_common::kurbo::{BezPath, Rect, Shape, Stroke};
 use vello_common::peniko::{Color, ColorStop, Fill, Gradient};
+use vello_common::pixmap::Pixmap;
 use vello_cpu::color::palette::css::RED;
 use vello_cpu::peniko::Compose;
+use vello_cpu::{Level, RenderContext, RenderMode, RenderSettings};
 use vello_dev_macros::vello_test;
 
 #[vello_test(width = 8, height = 8)]
@@ -374,3 +376,20 @@
     ]));
     ctx.fill_rect(&viewport);
 }
+
+#[test]
+fn multi_threading_oob_access() {
+    let settings = RenderSettings {
+        level: Level::new(),
+        num_threads: 4,
+    };
+    let mut ctx = RenderContext::new_with(100, 100, &settings);
+    let mut pixmap = Pixmap::new(100, 100);
+    
+    ctx.fill_path(&Rect::new(0.0, 0.0, 50.0, 50.0).to_path(0.1));
+    ctx.flush();
+    ctx.render_to_pixmap(&mut pixmap, RenderMode::OptimizeQuality);
+    ctx.fill_path(&Rect::new(50.0, 50.0, 100.0, 100.0).to_path(0.1));
+    ctx.flush();
+    ctx.render_to_pixmap(&mut pixmap, RenderMode::OptimizeQuality);
+}
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