fix: improve compute GSplat tile entry buffer scaling to reduce overflow artifacts

[mvaligursky]

Improves the dynamic tile entry buffer scaling in the compute GSplat renderer to reduce visual artifacts (missing splats) caused by buffer overflow when the camera rapidly reveals many close-up splats.

Changes:

• Increase grow headroom from 10% to 50% (ENTRY_HEADROOM_MULTIPLIER = 1.5) so the buffer has more room after an overflow-triggered resize
• Use gradual multiplier shrink (10% decay per cycle) instead of snapping directly to measured usage, preserving memory of past peak demand
• Increase SHRINK_THRESHOLD from 10 to 200 consecutive low-usage readbacks before considering a shrink, keeping existing buffer capacity longer

Performance:

• Temporarily higher peak memory for tile entry buffers, reclaimed gradually (~27s at 60fps) after sustained low usage
• Reduces multi-frame rendering artifacts when the camera reveals dense splat regions