Fix ant rendering GLSL version mismatch causing phantom food carry

The ant rendering ShaderMaterial was missing glslVersion: THREE.GLSL3
while every other material in the project had it. The vert shader used
varying (GLSL1) and the frag used in (GLSL3), causing vIsCarryingFood
to receive garbage values — making all ants appear to carry food from
the first frame regardless of actual state.

Also reverts the debug +20 cell spawn height offset.

src/scenes/ScreenScene.ts

@@ -99,6 +99,7 @@ export default class ScreenScene extends AbstractScene {
             vertexShader: vertexShaderAnts,
             fragmentShader: fragmentShaderAnts,
             transparent: true,
+            glslVersion: THREE.GLSL3,
         });
 
         this.createInstancedAntsMesh();

src/shaders/ants.vert

@@ -3,8 +3,8 @@ precision highp int;
 
 #define PI 3.1415926535897932384626433832795
 
-varying vec2 vUv;
+out vec2 vUv;
-varying float vIsCarryingFood;
+out float vIsCarryingFood;
 
 uniform sampler2D tData;