Mention the need for Flip Faces when using QuadMesh for post-processing (#6818)

Co-authored-by: Hugo Locurcio <[email protected]>

tutorials/shaders/advanced_postprocessing.rst

@@ -33,13 +33,14 @@ shader and to access the depth texture of the scene. Next, use a vertex shader
 to make the quad cover the screen at all times so that the post-processing
 effect will be applied at all times, including in the editor.
 
-First, create a new MeshInstance3D and set its mesh to a QuadMesh. This creates a quad
-centered at position ``(0, 0, 0)`` with a width and height of ``1``. Set the width
-and height to ``2``. Right now, the quad occupies a position in world space at the
-origin; however, we want it to move with the camera so that it always covers the
-entire screen. To do this, we will bypass the coordinate transforms that translate
-the vertex positions through the difference coordinate spaces and treat the vertices
-as if they were already in clip space.
+First, create a new MeshInstance3D and set its mesh to a QuadMesh. This creates
+a quad centered at position ``(0, 0, 0)`` with a width and height of ``1``. Set
+the width and height to ``2`` and enable **Flip Faces**. Right now, the quad
+occupies a position in world space at the origin. However, we want it to move
+with the camera so that it always covers the entire screen. To do this, we will
+bypass the coordinate transforms that translate the vertex positions through the
+difference coordinate spaces and treat the vertices as if they were already in
+clip space.
 
 The vertex shader expects coordinates to be output in clip space, which are coordinates
 ranging from ``-1`` at the left and bottom of the screen to ``1`` at the top and right