Update instructions on setting Full Rect anchor in Custom post-processing

This now contains a screenshot that shows how to set the anchor preset.

This also renames the `half` variable in the example shader to avoid
highlighting `half` as a GLSL language keyword.

Author: Calinou

tutorials/shaders/custom_postprocessing.rst

@@ -23,7 +23,13 @@ Post-processing effects are shaders applied to a frame after Godot has rendered
 it. To apply a shader to a frame, create a :ref:`CanvasLayer
 <class_CanvasLayer>`, and give it a :ref:`ColorRect <class_ColorRect>`. Assign a
 new :ref:`ShaderMaterial <class_ShaderMaterial>` to the newly created
-``ColorRect``, and set the ``ColorRect``'s layout to "Full Rect".
+``ColorRect``, and set the ``ColorRect``'s anchors preset to Full Rect:
+
+.. figure:: img/custom_postprocessing_anchors_preset_full_rect.webp
+   :align: center
+   :alt: Setting the anchors preset to Full Rect on the ColorRect node
+
+   Setting the anchors preset to Full Rect on the ColorRect node
 
 Your scene tree will look something like this:
 
@@ -67,8 +73,8 @@ shader by `arlez80 <https://bitbucket.org/arlez80/hex-mosaic/src/master/>`_,
 
     void fragment() {
             vec2 norm_size = size * SCREEN_PIXEL_SIZE;
-            bool half = mod(SCREEN_UV.y / 2.0, norm_size.y) / norm_size.y < 0.5;
-            vec2 uv = SCREEN_UV + vec2(norm_size.x * 0.5 * float(half), 0.0);
+            bool less_than_half = mod(SCREEN_UV.y / 2.0, norm_size.y) / norm_size.y < 0.5;
+            vec2 uv = SCREEN_UV + vec2(norm_size.x * 0.5 * float(less_than_half), 0.0);
             vec2 center_uv = floor(uv / norm_size) * norm_size;
             vec2 norm_uv = mod(uv, norm_size) / norm_size;
             center_uv += mix(vec2(0.0, 0.0),
@@ -78,7 +84,7 @@ shader by `arlez80 <https://bitbucket.org/arlez80/hex-mosaic/src/master/>`_,
                                  mix(vec2(0.0, -norm_size.y),
                                      vec2(-norm_size.x, -norm_size.y),
                                      float(norm_uv.x < 0.5)),
-                                 float(half)),
+                                 float(less_than_half)),
                              float(norm_uv.y < 0.3333333) * float(norm_uv.y / 0.3333333 < (abs(norm_uv.x - 0.5) * 2.0)));
 
             COLOR = textureLod(screen_texture, center_uv, 0.0);