Rework texture streaming and tracking.

This is a big one:
- Reworks the discard signal almost entirely.  Now has a normalized 0..1 discard signal: distance x size x channel exponent, floored by staleness and background app state.  Shaped by VRAM pressure.
- Textures can now scale down to the smallest GPU mip (1×1), independent of the codec's encoded mip
  count.
- Terrain texture LOD now works.  Useful for 2K textures and PBR on terrain.  Based upon camera distance to nearest terrain patch.
- New texture quality setting.  Low/Medium/High/Ultra
  - Caps texture resolution on Low to 1024, and otherwise shifts the discard signal around.  Makes distance based texture LOD work a lot more predictably.
- We now track last bind state for textures, and discard accordingly.  We progressively discard based upon last bind time.
- Avatar textures get a residency boost to stay loaded in VRAM longer under pressure.

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