hid/gtk: Focus file save dialog at exit to "Save", not "Close without saving".

Avoids inadvertently loosing work when closing the application. "Cancel" would
also be a safe choice of default focus, but the choice of "Save" is copied from
gedit to help give better platform consistency on Gnome systems.
png: add explicit round() calls to the scaling

After this commit the tests pass on both 32- and 64-bit builds.
Hopefully this time it's really dead; the previous "fix" af27c2d
by myself merely increased the DPI of the output, but this one
attempts to get deterministic rounding behaviour.

Tests from folks with actual 32-bit machines are welcome.

Closes-bug: lp-860037