Route remaining canvas GL constants through renderer gl

docs/modernization/build-inventory.md

@@ -226,6 +226,11 @@ Known local toolchain state:
   unbind targets, and merge framebuffer copy targets.
   Canvas equirectangular import drawing and depth export rendering also consume
   backend-owned depth/blend state and active texture units.
+  Canvas thumbnail generation and object-drawing helpers also consume
+  backend-owned saved viewport/clear/blend state, active texture units,
+  readback format/type, framebuffer copy targets, and renderbuffer/depth
+  attachment parameters; `src/canvas.cpp` no longer contains raw `GL_*`
+  constants.
 - `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.