EvSidebarThumbnails: render thumbnails correctly for hi-dpi displays

Render thumbnails with resolution from gtk_widget_get_scale_factor() and
recreate them when the scale change. Make the thumbnail-rendering helper
functions create a hi-resolution surface when the widget has a scale
factor.

https://bugzilla.gnome.org/show_bug.cgi?id=723431
libview: Fix g_return condition in set_device_scale_on_surface()

Fail when device scale is not 1 if hidpi is not available, we were doing
the opposite.