- tile_for_depth: alpha always 1.0, lerp RGB from white toward sky RGB
instead of fading tile out via alpha. Tiles stay opaque.
- Expose LAYER_ALPHA as pub const so it can be tuned at runtime
- LAYER_ALPHA changed from 41/255 to 50/255 for steeper visible fade
- Invisible tiles: explicit solid sky colour, not tile_for_depth(0, 999)
Replace df_fade_color (multiplicative tint that preserves texture hue)
with tile_for_depth which cross-fades between real tile and sky tile:
- z_diff <= 0: real tile at full opacity (no fade)
- z_diff in (0, threshold): real tile fades out via alpha
- z_diff past threshold: sky tile (index 0) fades in
- invisible tiles: solid sky colour
Also fix tile_id_to_tileset_index to include sky (id=0) as index 0,
so sky tiles render correctly as the fade target.
popertots