Route RTT region readbacks through backend

docs/modernization/roadmap.md

@@ -810,6 +810,10 @@ resource lifecycle path behind the renderer backend boundary.
 Legacy `RTT` resize/copy blits and byte/float framebuffer readbacks now execute
 through tested `pp_renderer_gl` framebuffer dispatch contracts with draw/read
 framebuffer binding restore handled by the backend helper.
+Legacy `RTT` also exposes an RGBA8 region-readback helper that uses the same
+backend framebuffer readback dispatch; canvas pick/history/snapshot and
+transform history paths now call that helper instead of binding an RTT and
+calling `glReadPixels` directly.
 Legacy `RTT::bindFramebuffer` and `RTT::unbindFramebuffer` now use tested
 `pp_renderer_gl` draw/read framebuffer binding snapshot and restore contracts,
 moving render-target pass entry/exit state management behind the backend.
@@ -1975,8 +1979,9 @@ Results:
   and fallback 2D texture unbinds through the renderer GL backend mapping;
   platform VR SDK bridges remain isolated for later platform-shell extraction.
 - Canvas mode overlay, mask, and transform paths now route generic OpenGL
-  blend/depth state, active texture units, 2D copy targets, and RGBA8
-  readback formats through the renderer GL backend mapping.
+  blend/depth state, active texture units, 2D copy targets, RGBA8 readback
+  formats, and RTT-backed transform history region readbacks through the
+  renderer GL backend mapping.
 - `NodeCanvas` panorama UI rendering now routes sampler defaults, saved
   viewport/clear/blend/depth/scissor state, color clears, active texture units,
   fallback 2D texture unbinds, copy targets, and RGBA8 render-target formats
@@ -1992,7 +1997,9 @@ Results:
   renderer GL backend mapping.
 - 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.
+  compositing copy targets through the renderer GL backend mapping; the
+  RTT-backed dirty-region readbacks now execute through the retained `RTT`
+  region-readback helper rather than direct `glReadPixels`.
 - Canvas layer merge rendering and explicit layer-merge compositing now route
   depth/blend state, active texture units, fallback 2D texture unbinds, and
   merge framebuffer copy targets through the renderer GL backend mapping.