Color player name labels by player type

Name text fill now darkens based on player type so human players stand
out from AI: human = black, nation = a bit gray, bot = greyer. Shades are
tunable in render-settings.json (nameShadeNation, nameShadeBot; human is
always 0).

Repurpose the previously-unused pd3.z slot (was isHuman, dead in the
fragment shader) to carry a per-player grayscale shade, and use it as the
name fill color directly so it applies in both day and night.

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

@@ -202,6 +202,8 @@
     "outlineB": 0.0,
     "outlineUsePlayerColor": true,
     "fillUsePlayerColor": false,
+    "nameShadeNation": 0.3,
+    "nameShadeBot": 0.4,
     "emojiRowOffset": 1.4,
     "statusRowOffset": 1.4
   },