Move canvas layer pixel mapping to renderer gl

docs/modernization/build-inventory.md

@@ -179,6 +179,9 @@ Known local toolchain state:
   and blend-state tokens.
   Simple UI text, text-input, border, scroll, and animation timeline draw
   paths also consume backend-owned blend-state tokens.
+  Canvas layer cube/equirect generation, clear, restore, and snapshot paths
+  also consume backend-owned cube/2D texture targets, active texture units,
+  blend/clear state, and RGBA8 read/write pixel mapping.
 - `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.