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/RenderSettings.ts

@@ -81,6 +81,7 @@ export interface RenderSettings {
   };
   railroad: {
     railMinZoom: number;
+    railFadeRange: number;
     railDetailZoom: number;
     railAlpha: number;
   };