Add minContribution property for compute GSplat contribution culling

[mvaligursky]

Summary

• Adds a new minContribution property to GSplatParams that culls splats whose total visual contribution (opacity × projected area) falls below a user-controllable threshold
• Replaces the previous hardcoded depth-dependent culling, giving users direct control over the quality/performance trade-off
• Applies only to the compute renderer (GSPLAT_RENDERER_COMPUTE); default value of 2.0 yields ~32% frame time reduction on dense scenes with very small visible quality loss

Public API Changes

New GSplatParams.minContribution property:

gsplat.minContribution = 2.0; // default — good quality/performance balance
gsplat.minContribution = 0.0; // disable culling entirely
gsplat.minContribution = 5.0; // more aggressive culling for mobile/perf