vertex shader: mirror vertices over x- and y-axes

gloom-rs/shaders/simple.vert

@@ -2,7 +2,15 @@
 
 in vec3 position;
 
+// this matrix mirrors over the x-axis and the y-axis simultaneously
+uniform mat4 flip = mat4(
+    vec4(-1.0f,  0.0f,  0.0f,  0.0f),  // column 0
+    vec4( 0.0f, -1.0f,  0.0f,  0.0f),  // column 1
+    vec4( 0.0f,  0.0f,  1.0f,  0.0f),  // column 2
+    vec4( 0.0f,  0.0f,  0.0f,  1.0f)   // column 3
+);
+
 void main()
 {
-    gl_Position = vec4(position, 1.0f);
-}
+    gl_Position = flip * vec4(position, 1.0f);
+}