[Lemiort]

Closes #27.

Summary

• require mesh shaders to expose an active s_vPosition attribute and fail with a clear exception otherwise
• configure, enable, and disable normal/UV/tangent attributes only when their locations are valid
• remove the previous test workaround that kept optional spotlight attributes artificially active
• add coverage for rejected missing positions and stencil draws with optimized-out optional inputs

Verification

• cmake --build build/build/Release --target NEE opengl_render_tests
• ctest --test-dir build/build/Release -R OpenGLRender --output-on-failure (9/9 passed)
• ctest --test-dir build/build/Release --output-on-failure --no-tests=error (36/36 passed)