Isolate draw merge background shader setup

docs/modernization/roadmap.md

@@ -2988,6 +2988,10 @@ Results:
   for its source-over-destination `CompDraw` uniform setup, while keeping the
   existing layer dirty mutation, framebuffer copy, texture binding, and draw
   order local to the Canvas layer-merge path.
+- `Canvas::draw_merge` now shares a retained draw-merge shader setup helper for
+  checkerboard backgrounds and the final merged texture redraw over the grid;
+  layer traversal, blend destination copies, sampler/texture binding, and draw
+  ordering remain in the legacy Canvas path.
 - Canvas thumbnail layer blending now uses the same canvas destination-feedback
   plan for framebuffer-fetch versus texture-copy decisions; the thumbnail draw
   itself still executes through retained OpenGL canvas code under DEBT-0036.