Do not serve WebP images that are larger than the original

The current code assumes the .webp is always smaller, but occasionally it isn't. Optimiser plugins may report this as "already optimised" or similar.

inc/Addon/WebP/Subscriber.php

@@ -435,14 +435,20 @@ private function url_to_webp( $url, $extensions ) {
 
 		if ( $this->filesystem->exists( $src_path_webp ) ) {
 			// File name: image.jpg => image.webp.
-			return preg_replace( '@\.' . $src_url['extension'] . '$@', '.webp', $src_url['src'] ) . $src_url['query'];
+			$src_url_webp = preg_replace( '@\.' . $src_url['extension'] . '$@', '.webp', $src_url['src'] ) . $src_url['query'];
+		} else if ( $this->filesystem->exists( $src_path_webp = $src_path . '.webp' ) ) {
+			// File name: image.jpg => image.jpg.webp.
+			$src_url_webp = $src_url['src'] . '.webp' . $src_url['query'];
+		} else {
+			// No webp exists
+			return false;
 		}
 
-		if ( $this->filesystem->exists( $src_path . '.webp' ) ) {
-			// File name: image.jpg => image.jpg.webp.
-			return $src_url['src'] . '.webp' . $src_url['query'];
+		if ( $this->filesystem->size( $src_path_webp ) < $this->filesystem->size( $src_path ) ) {
+			return $src_url_webp;
 		}
 
+		// The webp isn't smaller than the original, so don't use it.
 		return false;
 	}