add constant assertion that composite original mask bit is free

sparse_strips/vello_hybrid/src/filter.rs

diff --git a/sparse_strips/vello_hybrid/src/filter.rs b/sparse_strips/vello_hybrid/src/filter.rs
index 34cf99b..93b69e0 100644
--- a/sparse_strips/vello_hybrid/src/filter.rs
+++ b/sparse_strips/vello_hybrid/src/filter.rs
@@ -34,6 +34,7 @@
 const FILTER_SIZE_BYTES: usize = 48;
 const FILTER_SIZE_U32: usize = FILTER_SIZE_BYTES / 4;
 const COMPOSITE_ORIGINAL_SHIFT: u32 = 13;
+const COMPOSITE_ORIGINAL_MASK: u32 = 1 << COMPOSITE_ORIGINAL_SHIFT;
 
 const _: () = assert!(
     size_of::<GpuFilterData>() == FILTER_SIZE_BYTES,
@@ -97,7 +98,7 @@
     filter_type
 }
 
-fn pack_header_with_gaussian_params(
+const fn pack_header_with_gaussian_params(
     filter_type: u32,
     edge_mode: u32,
     n_decimations: u32,
@@ -111,6 +112,11 @@
     filter_type | (edge_mode << 5) | (n_decimations << 7) | (n_linear_taps << 11)
 }
 
+const _: () = assert!(
+    pack_header_with_gaussian_params(31, 3, 15, 3) & COMPOSITE_ORIGINAL_MASK == 0,
+    "Gaussian filter parameters overlap the composite_original bit"
+);
+
 // To a large degree, the vello_hybrid implementation of gaussian blur follows the one in vello_cpu.
 // However, we apply a specific optimization, where instead of averaging and weighting each sample
 // one after the other, we use linear sampling to sample two pixels at once, and adjust the
@@ -280,13 +286,19 @@
     )]
     fn from(shadow: &DropShadow) -> Self {
         let lk = LinearKernel::new(&shadow.kernel, shadow.kernel_size);
+        let composite_original = if shadow.composite_original {
+            COMPOSITE_ORIGINAL_MASK
+        } else {
+            0
+        };
+
         Self {
             header: pack_header_with_gaussian_params(
                 filter_type::DROP_SHADOW,
                 edge_mode_to_gpu(shadow.edge_mode),
                 shadow.n_decimations as u32,
                 lk.n_taps as u32,
-            ) | (u32::from(shadow.composite_original) << COMPOSITE_ORIGINAL_SHIFT),
+            ) | composite_original,
             center_weight: lk.center_weight,
             linear_weights: lk.weights,
             linear_offsets: lk.offsets,
@@ -321,7 +333,7 @@
     }
 
     pub(crate) fn composite_original(&self) -> bool {
-        (self.data[0] >> COMPOSITE_ORIGINAL_SHIFT) & 1 != 0
+        self.data[0] & COMPOSITE_ORIGINAL_MASK != 0
     }
 
     pub(crate) fn needs_copy_pass(&self) -> bool {