ResourceLoaderImage: Point to the right skin when generating URL

Previously, in skins using non-default OOUI theme (for example,
in MonoBook skin which uses Apex theme), when using a browser that
we don't serve SVG images to (for example, Internet Explorer 8),
the user would see the default theme's icons. This is noticeable
e.g. with the search box's icon on Special:Search.

Change-Id: I5a563c59efb267d8080161271513c0cc7d90d610

includes/resourceloader/ResourceLoaderImage.php

@@ -176,6 +176,7 @@ class ResourceLoaderImage {
 			'variant' => $variant,
 			'format' => $format,
 			'lang' => $context->getLanguage(),
+			'skin' => $context->getSkin(),
 			'version' => $context->getVersion(),
 		];