Extract fill modes, preview runtime pockets, and brush item UI

src/legacy_node_stroke_preview_runtime_services.h

@@ -12,9 +12,29 @@
 #include <cmath>
 #include <functional>
 #include <memory>
+#include <vector>
 
 namespace pp::panopainter {
 
+struct LegacyNodeStrokePreviewBackgroundCaptureRequest {
+    glm::vec2 size {};
+    bool colorize = false;
+    Texture2D& background_texture;
+    std::function<void()> draw_checkerboard;
+};
+
+void execute_legacy_node_stroke_preview_background_capture_pass(
+    const LegacyNodeStrokePreviewBackgroundCaptureRequest& request);
+
+struct LegacyNodeStrokePreviewStrokeComputeRequest {
+    const Stroke& stroke;
+    float zoom = 1.0f;
+    glm::vec2 mixer_size {};
+};
+
+[[nodiscard]] std::vector<LegacyNodeStrokePreviewFrame> plan_legacy_node_stroke_preview_stroke_frames(
+    const LegacyNodeStrokePreviewStrokeComputeRequest& request);
+
 struct LegacyNodeStrokePreviewImmediateRuntimeRequest {
     const std::shared_ptr<Brush>& brush;
     glm::vec2 preview_size {};