Rollup of fixes and improvements for cloud rendering

Some changes were made to cloud rendering in newer versions
that needed to be replicated in Sodium.

- The alpha cutoff for clouds was changed to (a < 10).
- Texture loading can now gracefully fail, and it is
expected that rendering is skipped when this happens.
- The movement/positioning of clouds was slightly changed.
- The render pass system now needs to be told about
render target usages (fixes #2883).

This commit also improves mesh building time by around 35% on
a fast processor (AMD Ryzen HX AI 370) through various
micro-optimizations.