Fix overflow in clipping

sparse_strips/vello_common/src/coarse.rs

diff --git a/sparse_strips/vello_common/src/coarse.rs b/sparse_strips/vello_common/src/coarse.rs
index ac8ca43..3245aff 100644
--- a/sparse_strips/vello_common/src/coarse.rs
+++ b/sparse_strips/vello_common/src/coarse.rs
@@ -521,10 +521,10 @@
 
             // Calculate starting position and column for alpha mask
             let mut x = x0;
-            let mut col = (strip.alpha_idx / u32::from(Tile::HEIGHT)) as u16;
+            let mut col = strip.alpha_idx / u32::from(Tile::HEIGHT);
             let clip_x = clip_bbox.x0() * WideTile::WIDTH;
             if clip_x > x {
-                col += clip_x - x;
+                col += (clip_x - x) as u32;
                 x = clip_x;
             }
 
@@ -543,10 +543,10 @@
                 let cmd = CmdClipAlphaFill {
                     x: x_rel,
                     width: width as u32,
-                    alpha_idx: (col * Tile::HEIGHT) as usize,
+                    alpha_idx: col as usize * Tile::HEIGHT as usize,
                 };
                 x += width;
-                col += width;
+                col += width as u32;
 
                 // Apply the clip strip command and update state
                 self.get_mut(wtile_x, cur_wtile_y).clip_strip(cmd);

sparse_strips/vello_cpu/src/render.rs

diff --git a/sparse_strips/vello_cpu/src/render.rs b/sparse_strips/vello_cpu/src/render.rs
index 68931b4..35522e2 100644
--- a/sparse_strips/vello_cpu/src/render.rs
+++ b/sparse_strips/vello_cpu/src/render.rs
@@ -275,8 +275,9 @@
 
 #[cfg(test)]
 mod tests {
+    use core::iter;
     use crate::RenderContext;
-    use vello_common::kurbo::Rect;
+    use vello_common::kurbo::{Rect, Shape};
 
     #[test]
     fn reset_render_context() {
@@ -295,4 +296,14 @@
         assert!(ctx.strip_buf.is_empty());
         assert!(ctx.alphas.is_empty());
     }
+    
+    #[test]
+    fn clip_overflow() {
+        let mut ctx = RenderContext::new(100, 100);
+        
+        ctx.alphas.extend(iter::repeat(255).take(u16::MAX as usize + 1));
+        
+        ctx.clip(&Rect::new(20.0, 20.0, 180.0, 180.0).to_path(0.1));
+        ctx.finish();
+    }
 }