rendering: add DrawList picking and selection

[tritao]

Summary

Add GPU picking and selection support to the retained DrawList renderer.

• Extend retained state with the information required for picking and selection.
• Add the DrawList ID-buffer rendering path.
• Build pick lookup tables from retained command and element ranges.
• Resolve object and element identities from framebuffer coordinates.
• Render retained selection overlays.
• Route picking GL operations through Coin's context-aware GL glue.
• Add exact ID/identity and selection-overlay pixel coverage on a core-profile EGL context.

This layer provides retained picking and selection primitives. Manager pipeline policy remains in PR #33.

Stack

Layer 14/16, based on PR #50.