TinyMCE: ensure the image toolbar stays visible when the image is much wider than the editor.

Props iseulde.
Merges [31362] to the 4.1 branch.
Fixes #30696.

Built from https://develop.svn.wordpress.org/branches/4.1@31437


git-svn-id: http://core.svn.wordpress.org/branches/4.1@31418 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/tinymce/plugins/wpeditimage/plugin.js

@@ -238,7 +238,9 @@ tinymce.PluginManager.add( 'wpeditimage', function( editor ) {
 		left = boundaryMiddle - toolbarHalf;
 		left += iframePos.x;
 
-		if ( toolbarWidth >= windowWidth ) {
+		if ( boundary.left < 0 || boundary.right > iframeWidth ) {
+			left = iframePos.x + ( iframeWidth - toolbarWidth ) / 2;
+		} else if ( toolbarWidth >= windowWidth ) {
 			className += ' mce-arrow-full';
 			left = 0;
 		} else if ( ( left < 0 && boundary.left + toolbarWidth > windowWidth ) ||