[rs] Avoid array indexing in hello-triangle

wgpu/examples/hello-triangle/shader.vert

@@ -4,12 +4,7 @@ out gl_PerVertex {
     vec4 gl_Position;
 };
 
-const vec2 positions[3] = vec2[3](
-    vec2(0.0, -0.5),
-    vec2(0.5, 0.5),
-    vec2(-0.5, 0.5)
-);
-
 void main() {
-    gl_Position = vec4(positions[gl_VertexIndex], 0.0, 1.0);
+    vec2 position = vec2(gl_VertexIndex, (gl_VertexIndex & 1) * 2) - 1;
+    gl_Position = vec4(position, 0.0, 1.0);
 }