screen: Remove alt-tab thumbnails

gdk_pixbuf_scale_simple generates very nice thumbnails at the expense of
a very slow rendering. So slow that the alt-tab popup is not usable: it
needs 3.6 seconds to downscale 30 windows of dimension 1600x900 on an
fairly recent computer.

We remove the thumbnailing to makes alt-tab blazing fast again in the
composited mode.

https://bugzilla.gnome.org/show_bug.cgi?id=504729