Refactor Vulkan setup phase 3

shaders/square.vert

@@ -1,7 +1,11 @@
 #version 450
 
-layout(location = 0) in vec2 in_position;
+vec2 positions[3] = vec2[](
+    vec2(0.0, -0.5),
+    vec2(0.5, 0.5),
+    vec2(-0.5, 0.5)
+);
 
 void main() {
-  gl_Position = vec4(in_position, 0.0, 1.0);
+  gl_Position = vec4(positions[gl_VertexIndex], 0.0, 1.0);
 }