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.