Fixed a drastic performance blow caused by Overview Docker

When working on 32-bit images the creation of the preview is quite an
expensive operation, so we needed to do it a bit more rarely. The
situation was worsened by the fact that the docker was generating
a preview event when it was hidden, and in this hidden state its size
was "implementation defined", which might be 1000+ pixels in some cases.

The patch also fixes various small bugs of the docker, like garbage when
resizing it.