nautilus-file.c: Add transparent padding to thumbnails

Pad thumbnails with transparency such that the resulting image is square. This ensures that the pixmap has room for emblems even when the image has an extreme aspect ratio.

Closes #912 (closed)

Thumbnails are aligned with the bottom edge of the transparent square. The only visual difference as a result of this change is that rows of thumbnails in nautilus are all the same vertical size. Previously, a row of thumbnails for vertically small images could take up less vertical space. Now with the padding, all rows of thumbnails will take up the same vertical space regardless of the aspect ratio of the images.

This makes things a little more spacious vertically, as in the following screenshot:

But with the new views it makes no difference since all icons/thumbnails take up the same sized square region anyway: