[naga wgsl-out] Correctly include width in matrix constructions.

When generating WGSL for an `Expression::Compose` constructing a
matrix, consult `TypeInner::Matrix::width` when writing the
type name in the construction expression, rather than just always
writing `matNxM<f32>`.

Fixes #4681.

naga/src/back/wgsl/writer.rs

@@ -524,14 +524,14 @@ impl<W: Write> Writer<W> {
             TypeInner::Matrix {
                 columns,
                 rows,
-                width: _,
+                width,
             } => {
                 write!(
                     self.out,
-                    //TODO: Can matrix be other than f32?
-                    "mat{}x{}<f32>",
+                    "mat{}x{}<{}>",
                     back::vector_size_str(columns),
                     back::vector_size_str(rows),
+                    scalar_kind_str(crate::Scalar::float(width))
                 )?;
             }
             TypeInner::Pointer { base, space } => {