PhotosIndex: add dualMixed layout

This combines one landscape with one portrait.

models/PhotoMosaic.php

@@ -52,6 +52,7 @@ class PhotoMosaic
 			// Dual layouts
 			'dualLandscapes' => [Image::TYPE_LANDSCAPE, Image::TYPE_LANDSCAPE],
 			'dualPortraits' => [Image::TYPE_PORTRAIT, Image::TYPE_PORTRAIT],
+			'dualMixed' => [Image::TYPE_LANDSCAPE, Image::TYPE_PORTRAIT],
 
 			// Fallback layouts
 			'singleLandscape' => [Image::TYPE_LANDSCAPE],