Extract stroke mix setup shell

src/legacy_canvas_stroke_execution_services.h

@@ -340,6 +340,21 @@ struct LegacyCanvasStrokeMixPassResult {
     std::size_t composed_planes = 0;
 };
 
+struct LegacyCanvasStrokeMixPassSetup {
+    std::function<void()> begin;
+    std::function<void()> end;
+};
+
+[[nodiscard]] inline LegacyCanvasStrokeMixPassSetup make_legacy_canvas_stroke_mix_pass_setup(
+    std::function<void()> begin,
+    std::function<void()> end)
+{
+    return LegacyCanvasStrokeMixPassSetup {
+        .begin = std::move(begin),
+        .end = std::move(end),
+    };
+}
+
 [[nodiscard]] inline LegacyCanvasStrokeMixPassRequest make_legacy_canvas_stroke_mix_pass_request(
     std::string_view context,
     glm::vec2 resolution,