AquaReal — Sarp-style Metals + Rain Wetness/Puddles

Keeps the existing AquaReal implementation of:
- PBR-style metallic reflections
- rain wetness on exposed, mostly horizontal terrain
- patchy rain puddles
- screen-space puddle reflections
- wet-surface darkening

Compile fix:
The terrain shader previously included dither.glsl twice when both shadow
casting and MATERIAL_GLOSS were enabled. The second include is now guarded,
so exactly one copy is compiled in every relevant configuration.

This is a clean AquaReal/Sarp-style implementation, not a repackaged copy
of Sarp's source.
