feat(render): fade railroad overlay near min zoom

Replace the hard zoom cutoff in RailroadPass with a linear alpha fade
controlled by a new `railFadeRange` setting. Rails (and bridge pixels)
ramp from invisible at `railMinZoom - railFadeRange` to fully opaque at
`railMinZoom`, instead of popping in. Adds a uRailFade shader uniform
and a debug slider.

src/client/render/gl/render-settings.json

@@ -79,6 +79,7 @@
   },
   "railroad": {
     "railMinZoom": 4,
+    "railFadeRange": 2,
     "railDetailZoom": 6,
     "railAlpha": 1
   },