Switch to the classic rainbow quad instead of a white one

Uses hardcoded resolution for now.

frag.frag

@@ -3,7 +3,5 @@
 layout(location = 0) out vec4 outColor;
 
 void main() {
-  if(1 == 1) {
-    outColor = vec4(1.0);
-  }
+  outColor = vec4(vec2(gl_FragCoord)/vec2(1920., 1080.), 1.0, 1.0);
 }