Route VR stroke setup through helpers

docs/modernization/roadmap.md

@@ -3013,6 +3013,10 @@ Results:
   now share the stroke erase shader setup helper, while texture binding, mask
   RTT binding, dirty/layer mutation, framebuffer feedback, and draw execution
   remain in retained Canvas code.
+- Desktop VR temporary erase/draw compositing now shares the retained stroke
+  erase and composite shader setup helpers, while sampler/texture binding,
+  per-eye/view transforms, temporary stroke texture selection, 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