Share retained stroke preview mix executor

docs/modernization/roadmap.md

@@ -3150,6 +3150,11 @@ Results:
   framebuffer bind/unbind, viewport/scissor/blend state, texture-slot
   binding, and final plane draw, while material planning and shader uniform
   setup remain in the preview node.
+- `NodeStrokePreview::stroke_draw_mix()` now routes retained mix-pass shader
+  setup plus framebuffer/state/input/draw ordering through
+  `execute_legacy_node_stroke_preview_mix_pass(...)`, while the preview node
+  keeps only the concrete GL save/restore, texture-object bind, and plane-draw
+  callbacks.
 - `pp_paint_renderer_stroke_execution_tests` now covers retained stroke texture
   input binding order, sample execution destination-copy behavior, live-pass
   face-framebuffer dirty tracking, and pad-face destination-copy behavior