Optimize indexed face rendering with adaptive material strategies

[tritao]

Improve SoIndexedFaceSet rendering for opaque PER_FACE_INDEXED materials.

The renderer now analyzes referenced face materials and selects an appropriate strategy:

• overall material for uniform referenced colors;
• grouped material indexers for a small number of repeated colors;
• unified coordinate/normal/color/index VBOs for high-cardinality materials;
• existing fallback rendering for unsupported cases.

This is intended to reduce per-face material overhead while preserving indexed geometry and Coin-owned VBO management.

Related to FreeCAD/FreeCAD#31603.

Implementation

• Analyze only materials referenced by the current face set.
• Group faces by resolved packed color rather than material index.
• Deduplicate unified vertices by coordinate, normal, and packed color.
• Keep coordinate, normal, color, and index buffers separately cacheable.
• Correct packed RGBA byte order for little-endian systems.
• Reset Coin’s cached diffuse state after color-array rendering.
• Include source node generations, pointers, counts, and color hashes in cache keys.
• Preserve the referenced material when a uniform face set uses a nonzero material index.
• Fall back for transparency, textures, custom vertex attributes, invalid material data, and unsupported topology.

The strategy selector and crossover thresholds are configurable through diagnostic environment variables:

COIN_FACE_MATERIAL_STRATEGY=auto|overall|grouped|unified|fallback
COIN_FACE_MATERIAL_MAX_GROUPS
COIN_FACE_MATERIAL_MIN_TRIANGLES_PER_GROUP
COIN_FACE_MATERIAL_MIN_UNIFIED_TRIANGLES
COIN_FACE_MATERIAL_MAX_UNIFIED_BYTES

Rendering statistics can optionally be written as JSON with:

COIN_RENDER_STATS_OUTPUT=<path>

Tests

Added Coin-native offscreen regression coverage for:

• uniform nonzero material indices;
• multiple material indices resolving to the same packed color;
• asymmetric packed RGBA;
• forced overall, grouped, unified, and fallback strategies;
• automatic fallback for small high-cardinality geometry;
• color, coordinate, normal, material-index, and coordinate-index mutations;
• unified rejection for textures and custom vertex attributes;
• transparent material fallback;
• unified cache hits and rebuilds.