fix(retro): CRT vertex shader failed to compile, rendering the 3D canvas black

vec4(position, 0.0, 1.0) passes a vec3 plus two floats into a vec4 - five
components into four. Invalid GLSL ES on both WebGL 1 and 2:

  THREE.WebGLProgram: Shader Error 0 - VALIDATE_STATUS false
  ERROR: 0:71: 'constructor' : too many arguments

The whole post-processing pass failed and the game canvas read back as one
colour, 0/64000 non-black pixels. position.xy is the vec2 the fullscreen quad
needs.

Verified headlessly with SwiftShader: 55-143 distinct colours, 63750/64000
lit pixels, no shader errors, scene hash changes after throttle + pitch.

js/retro.js

@@ -26,7 +26,7 @@ export class RetroEffect {
           varying vec2 vUv;
           void main() {
             vUv = uv;
-            gl_Position = vec4(position, 0.0, 1.0);
+            gl_Position = vec4(position.xy, 0.0, 1.0);
           }
         `,
         fragmentShader: `