Introducing the maximum size preview

The first time we're asked to generate a preview we'll generate one of the maximum dimension indicated in the configuration and all future resizing requests will be done on that preview in order to not waste time converting the same file over and over.

One of the fixes required for #12465

Author: oparoz

config/config.sample.php

@@ -620,12 +620,12 @@
  * The maximum width, in pixels, of a preview. A value of ``null`` means there
  * is no limit.
  */
-'preview_max_x' => null,
+'preview_max_x' => 2048,
 /**
  * The maximum height, in pixels, of a preview. A value of ``null`` means there
  * is no limit.
  */
-'preview_max_y' => null,
+'preview_max_y' => 2048,
 /**
  * If a lot of small pictures are stored on the ownCloud instance and the
  * preview system generates blurry previews, you might want to consider setting