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
1 job for master in 12 minutes and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #303156
flatpak devel

00:12:34