Refine stroke mix pass boundary

src/legacy_canvas_stroke_execution_services.h

@@ -179,6 +179,45 @@ struct LegacyCanvasStrokeMixPassResult {
     std::size_t composed_planes = 0;
 };
 
+[[nodiscard]] inline LegacyCanvasStrokeMixPassResult execute_legacy_canvas_stroke_mix_pass(
+    const LegacyCanvasStrokeMixPassRequest& request);
+
+template <std::size_t PlaneCount, typename HasTarget>
+[[nodiscard]] inline std::array<LegacyCanvasStrokeMixPassPlane, PlaneCount>
+plan_legacy_canvas_stroke_mix_pass_planes(
+    bool visible,
+    float opacity,
+    const glm::mat4& mvp_base,
+    const std::array<glm::mat4, PlaneCount>& plane_transforms,
+    HasTarget&& has_target,
+    const glm::mat4& plane_offset = glm::translate(glm::vec3(0.0f, 0.0f, -1.0f)))
+{
+    std::array<LegacyCanvasStrokeMixPassPlane, PlaneCount> planes {};
+    for (std::size_t plane_index = 0; plane_index < PlaneCount; ++plane_index) {
+        planes[plane_index] = LegacyCanvasStrokeMixPassPlane {
+            .index = static_cast<int>(plane_index),
+            .visible = visible,
+            .has_target = static_cast<bool>(has_target(static_cast<int>(plane_index))),
+            .opacity = opacity,
+            .mvp = mvp_base * plane_transforms[plane_index] * plane_offset,
+        };
+    }
+    return planes;
+}
+
+template <typename BeginMixPass, typename EndMixPass>
+[[nodiscard]] inline LegacyCanvasStrokeMixPassResult
+execute_legacy_canvas_stroke_mix_pass_with_setup(
+    BeginMixPass&& begin_mix_pass,
+    EndMixPass&& end_mix_pass,
+    const LegacyCanvasStrokeMixPassRequest& request)
+{
+    begin_mix_pass();
+    const auto result = execute_legacy_canvas_stroke_mix_pass(request);
+    end_mix_pass();
+    return result;
+}
+
 struct LegacyCanvasStrokeComputeRequest {
     StrokeSample previous_sample {};
     std::span<const StrokeSample> samples;