Fix texture mapping in Custom post-processing (#3957)

When the viewport is not fullscreen, the view is cropped and distorted after applying a shader.
This is due to using SCREEN_UV (meant for SCREEN_TEXTURE) instead of UV (meant for TEXTURE).

tutorials/viewports/custom_postprocessing.rst

@@ -77,15 +77,15 @@ Copy the following code to your shader. The above code is a single pass edge det
   shader_type canvas_item;
 
   void fragment() {
-      vec3 col = -8.0 * texture(TEXTURE, SCREEN_UV).xyz;
-      col += texture(TEXTURE, SCREEN_UV + vec2(0.0, SCREEN_PIXEL_SIZE.y)).xyz;
-      col += texture(TEXTURE, SCREEN_UV + vec2(0.0, -SCREEN_PIXEL_SIZE.y)).xyz;
-      col += texture(TEXTURE, SCREEN_UV + vec2(SCREEN_PIXEL_SIZE.x, 0.0)).xyz;
-      col += texture(TEXTURE, SCREEN_UV + vec2(-SCREEN_PIXEL_SIZE.x, 0.0)).xyz;
-      col += texture(TEXTURE, SCREEN_UV + SCREEN_PIXEL_SIZE.xy).xyz;
-      col += texture(TEXTURE, SCREEN_UV - SCREEN_PIXEL_SIZE.xy).xyz;
-      col += texture(TEXTURE, SCREEN_UV + vec2(-SCREEN_PIXEL_SIZE.x, SCREEN_PIXEL_SIZE.y)).xyz;
-      col += texture(TEXTURE, SCREEN_UV + vec2(SCREEN_PIXEL_SIZE.x, -SCREEN_PIXEL_SIZE.y)).xyz;
+      vec3 col = -8.0 * texture(TEXTURE, UV).xyz;
+      col += texture(TEXTURE, UV + vec2(0.0, SCREEN_PIXEL_SIZE.y)).xyz;
+      col += texture(TEXTURE, UV + vec2(0.0, -SCREEN_PIXEL_SIZE.y)).xyz;
+      col += texture(TEXTURE, UV + vec2(SCREEN_PIXEL_SIZE.x, 0.0)).xyz;
+      col += texture(TEXTURE, UV + vec2(-SCREEN_PIXEL_SIZE.x, 0.0)).xyz;
+      col += texture(TEXTURE, UV + SCREEN_PIXEL_SIZE.xy).xyz;
+      col += texture(TEXTURE, UV - SCREEN_PIXEL_SIZE.xy).xyz;
+      col += texture(TEXTURE, UV + vec2(-SCREEN_PIXEL_SIZE.x, SCREEN_PIXEL_SIZE.y)).xyz;
+      col += texture(TEXTURE, UV + vec2(SCREEN_PIXEL_SIZE.x, -SCREEN_PIXEL_SIZE.y)).xyz;
       COLOR.xyz = col;
   }
 
@@ -135,15 +135,15 @@ does not matter:
 
   // Blurs the screen in the X-direction.
   void fragment() {
-      vec3 col = texture(TEXTURE, SCREEN_UV).xyz * 0.16;
-      col += texture(TEXTURE, SCREEN_UV + vec2(SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.15;
-      col += texture(TEXTURE, SCREEN_UV + vec2(-SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.15;
-      col += texture(TEXTURE, SCREEN_UV + vec2(2.0 * SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.12;
-      col += texture(TEXTURE, SCREEN_UV + vec2(2.0 * -SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.12;
-      col += texture(TEXTURE, SCREEN_UV + vec2(3.0 * SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.09;
-      col += texture(TEXTURE, SCREEN_UV + vec2(3.0 * -SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.09;
-      col += texture(TEXTURE, SCREEN_UV + vec2(4.0 * SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.05;
-      col += texture(TEXTURE, SCREEN_UV + vec2(4.0 * -SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.05;
+      vec3 col = texture(TEXTURE, UV).xyz * 0.16;
+      col += texture(TEXTURE, UV + vec2(SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.15;
+      col += texture(TEXTURE, UV + vec2(-SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.15;
+      col += texture(TEXTURE, UV + vec2(2.0 * SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.12;
+      col += texture(TEXTURE, UV + vec2(2.0 * -SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.12;
+      col += texture(TEXTURE, UV + vec2(3.0 * SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.09;
+      col += texture(TEXTURE, UV + vec2(3.0 * -SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.09;
+      col += texture(TEXTURE, UV + vec2(4.0 * SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.05;
+      col += texture(TEXTURE, UV + vec2(4.0 * -SCREEN_PIXEL_SIZE.x, 0.0)).xyz * 0.05;
       COLOR.xyz = col;
   }
 
@@ -153,15 +153,15 @@ does not matter:
 
   // Blurs the screen in the Y-direction.
   void fragment() {
-      vec3 col = texture(TEXTURE, SCREEN_UV).xyz * 0.16;
-      col += texture(TEXTURE, SCREEN_UV + vec2(0.0, SCREEN_PIXEL_SIZE.y)).xyz * 0.15;
-      col += texture(TEXTURE, SCREEN_UV + vec2(0.0, -SCREEN_PIXEL_SIZE.y)).xyz * 0.15;
-      col += texture(TEXTURE, SCREEN_UV + vec2(0.0, 2.0 * SCREEN_PIXEL_SIZE.y)).xyz * 0.12;
-      col += texture(TEXTURE, SCREEN_UV + vec2(0.0, 2.0 * -SCREEN_PIXEL_SIZE.y)).xyz * 0.12;
-      col += texture(TEXTURE, SCREEN_UV + vec2(0.0, 3.0 * SCREEN_PIXEL_SIZE.y)).xyz * 0.09;
-      col += texture(TEXTURE, SCREEN_UV + vec2(0.0, 3.0 * -SCREEN_PIXEL_SIZE.y)).xyz * 0.09;
-      col += texture(TEXTURE, SCREEN_UV + vec2(0.0, 4.0 * SCREEN_PIXEL_SIZE.y)).xyz * 0.05;
-      col += texture(TEXTURE, SCREEN_UV + vec2(0.0, 4.0 * -SCREEN_PIXEL_SIZE.y)).xyz * 0.05;
+      vec3 col = texture(TEXTURE, UV).xyz * 0.16;
+      col += texture(TEXTURE, UV + vec2(0.0, SCREEN_PIXEL_SIZE.y)).xyz * 0.15;
+      col += texture(TEXTURE, UV + vec2(0.0, -SCREEN_PIXEL_SIZE.y)).xyz * 0.15;
+      col += texture(TEXTURE, UV + vec2(0.0, 2.0 * SCREEN_PIXEL_SIZE.y)).xyz * 0.12;
+      col += texture(TEXTURE, UV + vec2(0.0, 2.0 * -SCREEN_PIXEL_SIZE.y)).xyz * 0.12;
+      col += texture(TEXTURE, UV + vec2(0.0, 3.0 * SCREEN_PIXEL_SIZE.y)).xyz * 0.09;
+      col += texture(TEXTURE, UV + vec2(0.0, 3.0 * -SCREEN_PIXEL_SIZE.y)).xyz * 0.09;
+      col += texture(TEXTURE, UV + vec2(0.0, 4.0 * SCREEN_PIXEL_SIZE.y)).xyz * 0.05;
+      col += texture(TEXTURE, UV + vec2(0.0, 4.0 * -SCREEN_PIXEL_SIZE.y)).xyz * 0.05;
       COLOR.xyz = col;
   }