spice-widget: use nearest scaling for integral scaling factors

skullernet requested to merge skullernet/spice-gtk:nearest-scaling into master

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.

Disclaimer: I only tested this with Cairo, not EGL.

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