spice-widget: use nearest scaling for integral scaling factors

This makes text easier to read on HiDPI display if guest OS doesn't
support HiDPI scaling.

This also changes EGL widget minification filter from NEAREST to LINEAR
to make it behave the same as Cairo widget.

Fixes: #92.
2 jobs for nearest-scaling in 3 minutes and 35 seconds (queued for 8 seconds)