rendering: add DrawList picking and selection

[tritao]

Builds on #50.

Purpose

The DrawList renderer needs picking and selection to identify the same scene
objects and subelements that produced the visible frame. It also needs to do
that without attaching application selection policy to the renderer or
performing hidden picking work during ordinary drawing.

This PR carries object, face, edge, and vertex identity through retained
commands, renders exact integer IDs into a dedicated OpenGL pick target, and
resolves those IDs back into normal Coin pick results. Selection and highlight
overlays consume explicit frame-local targets derived from the same retained
identity.

What changed

• Retain compact command paths and structured object/subelement ranges.
• Build a frame-local lookup table with exact integer pick IDs.
• Render and query an R32UI picking target, including radius and layered-hit
  handling.
• Reuse the retained triangle, line, point, raster, and fallback geometry used
  by visual rendering.
• Render depth-tested, blended selection and highlight overlays from explicit
  frame-local targets.

Ownership and scope

Ordinary visual rendering does not perform picking implicitly. This layer
owns renderer-side identity, GPU picking, and overlay execution; it does not
own application identity, selection policy, manager orchestration, or render
layers.

Validation

• Object and face/edge/vertex picking.
• Stable scene identity through retained lookup and result resolution.
• Radius and layered GPU pick queries.
• Selection and highlight overlay rendering across retained geometry paths.