add iconGrowZoom for structures that scale with deep zoom

Previously structures capped at iconScale = 1.0 once zoomed past
iconScaleFactorZoomedOut, staying at a fixed pixel size no matter how
far you zoomed in. They felt overlaid on the map instead of part of it.

Add a third zoom band controlled by structure.iconGrowZoom. Past this
threshold iconScale = uZoom / iconGrowZoom — structures grow with the
canvas (world-anchored, fixed map-area coverage). Plumbed via the
uIconGrowZoom uniform on StructurePass.

Default 7 keeps normal play unchanged; only kicks in at deep zoom.

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

@@ -82,10 +82,11 @@
     "railAlpha": 1
   },
   "structure": {
-    "iconSize": 65,
+    "iconSize": 50,
     "dotsZoomThreshold": 1.2,
     "dotScale": 0.3,
-    "iconScaleFactorZoomedOut": 3.5,
+    "iconScaleFactorZoomedOut": 3,
+    "iconGrowZoom": 7,
     "shapes": {
       "City": {
         "scale": 1,