Route stroke frame-face traversal through helper

src/legacy_canvas_stroke_execution_services.h

@@ -8,6 +8,7 @@
 #include "util.h"
 
 #include <array>
+#include <cstddef>
 #include <functional>
 #include <span>
 #include <string_view>
@@ -152,6 +153,27 @@ template <typename ProjectStroke, typename MakeFrame>
     return frames;
 }
 
+template <typename Frames, typename BeginFrame, typename ExecuteFace>
+std::size_t execute_legacy_canvas_stroke_frame_faces(
+    Frames&& frames,
+    BeginFrame&& begin_frame,
+    ExecuteFace&& execute_face)
+{
+    std::size_t executed_faces = 0;
+    for (auto& frame : frames) {
+        begin_frame(frame);
+        for (int face_index = 0; face_index < 6; ++face_index) {
+            auto& vertices = frame.shapes[face_index];
+            if (vertices.size() < 3) {
+                continue;
+            }
+            execute_face(frame, face_index, vertices);
+            ++executed_faces;
+        }
+    }
+    return executed_faces;
+}
+
 [[nodiscard]] inline pp::paint_renderer::CanvasStrokeBox legacy_canvas_stroke_box(glm::vec4 box) noexcept
 {
     return pp::paint_renderer::CanvasStrokeBox {