Rewrite clouds2d.js to sample a noise texture instead of computing
fBm with hash functions. Each octave is one texture lookup with
doubled UV scale and halved amplitude.
- Same visual range as before but ~4x cheaper on ALU
- REPEAT-wrapped texture tiles seamlessly when scrolled by time
- Per-pixel hash21 jitter keeps the regular tiling from flickering
- Exposes noise as a uniform, so multiple fragment shaders could
share it
- Theme-agnostic: pulls colours from __currentPalette
Also adds #renderer=clouds2d URL-hash override for quick A/B
testing between the volumetric clouds3d and the 2D version.
Richard Zink
bfbb73a2af