Speed up sogs centers calc

[slimbuck]

This PR implements faster sogs centers generation.

This code is executed during scene load, so worth improving.

This change results in around 25% faster execution on a 4M gaussian test scene (468ms -> 343ms).

[Copilot]

Pull Request Overview

This PR improves the performance of SOGS centers generation during scene load by optimizing the per-iteration computations.

• Removed the per-iteration Vec3 allocation and created a more efficient loop using precomputed buffers.
• Introduced direct bit manipulation and math operations to calculate centers, reducing execution time by roughly 25%.

Comments suppressed due to low confidence (2)

src/scene/gsplat/gsplat-sogs-data.js:196

• [nitpick] The local variable 'means_u' shadows the destructured variable 'means_u' from earlier. Consider renaming it (e.g., 'mu_val' or 'upperMean') to improve code clarity.

const means_u = means_u_data[idx];

src/scene/gsplat/gsplat-sogs-data.js:197

• [nitpick] The local variable 'means_l' shadows the destructured variable 'means_l' from earlier. Consider renaming it (e.g., 'ml_val' or 'lowerMean') to improve code readability.

const means_l = means_l_data[idx];