Route thumbnail blend setup through helper

docs/modernization/roadmap.md

@@ -3024,6 +3024,10 @@ Results:
 - 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::thumbnail_generate` thumbnail layer blending now shares the retained
+  draw-merge shader setup helper for per-layer `TextureBlend` uniforms, while
+  destination feedback copies, sampler/texture binding, grid redraw, readback,
+  and draw execution remain in retained Canvas 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