GLSLNodeBuilder: Fix `texelFetch*()` code generation with depth textures. (#31760)

* Add a swizzle when sampling a depth texture

* Texture parameter can be null

* Rollback to using vec4 for depth texture UV

src/renderers/webgl-fallback/nodes/GLSLNodeBuilder.js

@@ -380,7 +380,7 @@ ${ flowData.code }
 	/**
 	 * Generates the GLSL snippet that reads a single texel from a texture without sampling or filtering.
 	 *
-	 * @param {Texture} texture - The texture.
+	 * @param {?Texture} texture - The texture.
 	 * @param {string} textureProperty - The name of the texture uniform in the shader.
 	 * @param {string} uvIndexSnippet - A GLSL snippet that represents texture coordinates used for sampling.
 	 * @param {?string} depthSnippet - A GLSL snippet that represents the 0-based texture array index to sample.
@@ -390,28 +390,42 @@ ${ flowData.code }
 	 */
 	generateTextureLoad( texture, textureProperty, uvIndexSnippet, depthSnippet, offsetSnippet, levelSnippet = '0' ) {
 
+		let snippet;
+
 		if ( depthSnippet ) {
 
 			if ( offsetSnippet ) {
 
-				return `texelFetchOffset( ${ textureProperty }, ivec3( ${ uvIndexSnippet }, ${ depthSnippet } ), ${ levelSnippet }, ${ offsetSnippet } )`;
+				snippet = `texelFetchOffset( ${ textureProperty }, ivec3( ${ uvIndexSnippet }, ${ depthSnippet } ), ${ levelSnippet }, ${ offsetSnippet } )`;
 
-			}
+			} else {
 
-			return `texelFetch( ${ textureProperty }, ivec3( ${ uvIndexSnippet }, ${ depthSnippet } ), ${ levelSnippet } )`;
+				snippet = `texelFetch( ${ textureProperty }, ivec3( ${ uvIndexSnippet }, ${ depthSnippet } ), ${ levelSnippet } )`;
+
+			}
 
 		} else {
 
 			if ( offsetSnippet ) {
 
-				return `texelFetchOffset( ${ textureProperty }, ${ uvIndexSnippet }, ${ levelSnippet }, ${ offsetSnippet } )`;
+				snippet = `texelFetchOffset( ${ textureProperty }, ${ uvIndexSnippet }, ${ levelSnippet }, ${ offsetSnippet } )`;
+
+			} else {
+
+				snippet = `texelFetch( ${ textureProperty }, ${ uvIndexSnippet }, ${ levelSnippet } )`;
 
 			}
 
-			return `texelFetch( ${ textureProperty }, ${ uvIndexSnippet }, ${ levelSnippet } )`;
+		}
+
+		if ( texture !== null && texture.isDepthTexture ) {
+
+			snippet += '.x';
 
 		}
 
+		return snippet;
+
 	}
 
 	/**