Code Modernization: Introduce is_gd_image() to check for PHP 8 GdImage object instances.

In PHP 8, the GD extension uses `GdImage` objects instead of resources for its underlying data structures.

This updates the existing `is_resource()` calls for image resources in core to accomodate for `GdImage` instances as well.

Props ayeshrajans, jrf.
Fixes #50833.
Built from https://develop.svn.wordpress.org/trunk@48798


git-svn-id: http://core.svn.wordpress.org/trunk@48560 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-image-editor-imagick.php

@@ -548,6 +548,7 @@ class WP_Image_Editor_Imagick extends WP_Image_Editor {
 		} catch ( Exception $e ) {
 			return new WP_Error( 'image_crop_error', $e->getMessage() );
 		}
+
 		return $this->update_size();
 	}
 
@@ -582,6 +583,7 @@ class WP_Image_Editor_Imagick extends WP_Image_Editor {
 		} catch ( Exception $e ) {
 			return new WP_Error( 'image_rotate_error', $e->getMessage() );
 		}
+
 		return true;
 	}