Route VR checkerboard setup through helper

docs/modernization/roadmap.md

@@ -3021,6 +3021,9 @@ Results:
   stroke composite shader setup helper, while render-task ordering, framebuffer
   copy bounds, sampler/texture binding, and draw execution remain in retained
   canvas-mode code.
+- Desktop VR checkerboard background drawing now shares the retained draw-merge
+  shader setup helper, while render state transitions, per-plane transforms,
+  and draw execution remain in retained VR code.
 - `Canvas::stroke_draw_mix` now shares the retained stroke composite shader
   helper for mixer-pass `CompDraw` setup, while preserving its caller-specific
   texture slot uniforms. Mixer framebuffer/scissor state, sampler and texture