Move canvas merge GL mappings to renderer gl

docs/modernization/build-inventory.md

@@ -221,6 +221,9 @@ Known local toolchain state:
   Canvas stroke commit also consumes backend-owned saved viewport/clear/blend
   state, history readback format/type, active texture units, fallback 2D
   texture unbind targets, and layer compositing copy targets.
+  Canvas layer merge rendering also consumes backend-owned depth/blend state,
+  active texture units, fallback 2D texture unbind targets, and merge
+  framebuffer copy targets.
 - `windows-msvc-vcpkg-headless` validates manifest install/configure/build/test
   for the current headless component matrix; see DEBT-0007 for remaining app
   and platform triplet migration.

docs/modernization/roadmap.md

@@ -765,6 +765,9 @@ Results:
 - Canvas stroke commit now routes saved viewport/clear/blend state, history
   readbacks, active texture units, fallback 2D texture unbinds, and layer
   compositing copy targets through the renderer GL backend mapping.
+- Canvas layer merge rendering now routes depth/blend state, active texture
+  units, fallback 2D texture unbinds, and merge framebuffer copy targets through
+  the renderer GL backend mapping.
 - Known remaining warnings: legacy project/vendor diagnostics, Visual Studio
   vcpkg-manifest warning, `LNK4099` missing libyuv PDBs, and `LNK4098` runtime
   library conflict from retained vendor binaries.