EXPOSURE / NIGHT / CAVE / BLOCK LIGHT SEPARATION

- EXPOSURE_MULTIPLIER is now global/base exposure only.
- Night lighting uses NIGHT_LIGHT_MULTIPLIER independently.
- Cave visibility uses CAVE_LIGHT_MULTIPLIER independently.
- Per-pixel Minecraft block light (torch/lantern/etc.) is written to colortex2
  and uses BLOCK_LIGHT_MULTIPLIER independently.
- composite1.vsh no longer folds night/cave boosts into the exposure metadata.
- colortex2 is cleared each frame so non-lightmapped pixels do not inherit stale data.
