chore: Simplify code using Cell::as_array_of_cells

Author: SuchAFuriousDeath

core/src/avm1/globals/color_matrix_filter.rs

@@ -61,10 +61,9 @@ impl<'gc> ColorMatrixFilter<'gc> {
     }
 
     fn matrix(self, activation: &mut Activation<'_, 'gc>) -> Value<'gc> {
-        // Use `.as_slice_of_cells()` to avoid a copy out of the `Cell`.
-        // FIXME: use `.as_array_of_cells()` once stabilized.
-        let matrix: &Cell<[f32]> = &self.0.matrix;
-        let matrix = matrix.as_slice_of_cells();
+        // Use `.as_array_of_cells()` to avoid a copy out of the `Cell`.
+        let matrix = self.0.matrix.as_array_of_cells();
+
         ArrayBuilder::new(activation)
             .with(matrix.iter().map(|v| v.get().into()))
             .into()