rendering: batch retained submissions

[tritao]

Builds on #69.

Purpose

Stable geometry resources remove repeated preparation, but a scene containing
many compatible occurrences can still issue one OpenGL draw call per retained
command. Submission overhead then grows with occurrence count even when the
commands use the same geometry and render state.

This PR lets the render plan group compatible commands and submit them as
instanced batches. The same per-instance identity is carried through visual
rendering, picking, and selection.

What changed

• Group compatible retained commands into shared GPU submissions.
• Dispatch instanced drawing through Coin's OpenGL glue layer.
• Carry shader-side draw and primitive identity through visual, picking, and
  selection shaders.
• Centralize compatibility and ordering decisions in internal command traits.
• Group only opaque commands that are safe to reorder.

Ordering contract

Batch compatibility is a semantic decision, not just a matching-shader test.
Transparent commands, render barriers, and depth-sensitive operations preserve
their required order. Geometry-resource ownership remains in #69; this layer
owns planner grouping and submission policy.

Validation

• Instanced visual rendering in compatibility and core profiles.
• Picking and selection preserve command and subelement identity.
• Planner tests cover safe opaque grouping and order-sensitive exclusions.
• Glue/runtime coverage checks the required instancing and ordered-submission
  capabilities.