shader/execution: Fix struct padding for f16s

`wgslMembers()` was using an `i32` to pad the structure to its alignment, however `i32` has an alignment and size of 4 bytes.
If the structure holds an odd number of `f16` scalars, then `i32` cannot be used to account fo this padding due to the 4-byte size and alignment.
Pad with `f16` if the padding is an odd multiple of 2 bytes.

src/webgpu/shader/execution/expression/expression.ts

@@ -158,7 +158,10 @@ function wgslMembers(members: Type[], source: InputSource, memberName: (i: numbe
   });
   const padding = layout.stride - layout.size;
   if (padding > 0) {
-    lines.push(`  @size(${padding}) padding : i32,`);
+    // Pad with a 'f16' if the padding requires an odd multiple of 2 bytes.
+    // This is required as 'i32' has an alignment and size of 4 bytes.
+    const ty = (padding & 2) !== 0 ? 'f16' : 'i32';
+    lines.push(`  @size(${padding}) padding : ${ty},`);
   }
   return lines.join('\n');
 }