[Lemiort]

Closes #23.

Problem

The deferred geometry pass replaced every command's surface material with one shared pass material. Objects therefore lost their diffuse, normal, and specular textures, and batching grouped instances according to the override instead of their actual material state.

Summary

• separate pass shader selection from surface material texture binding
• retain each render command's material when sorting and batching instances
• bind that material's diffuse, normal, specular, shadow, and custom textures against the deferred pass shader
• remove the redundant shared deferred geometry material
• add an OpenGL regression test with two materials and two instanced batches that verifies distinct diffuse/normal/specular values

Verification

• cmake --build build/build/Release --target NEE opengl_render_tests
• ctest --test-dir build/build/Release -R PassShaderPreservesPerInstanceSurfaceMaterials --output-on-failure
• ctest --test-dir build/build/Release -R OpenGLRender --output-on-failure (8/8 passed)
• ctest --test-dir build/build/Release --output-on-failure --no-tests=error (39/39 passed)