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
polygon.c: Readability cleanup for frac_circle()
Fix some broken assumptions regarding 32-bit integer coordinates in polygon1.c
Allow the coordinate position_hbox to expand and fill.

By allowing to fill and expand, shrinking becomes possible.

Signed-off-by: Bert Timmerman <bert.timmerman@xs4all.nl>
Changed the width of the relative coordinate label.

By changing the single line label into a multi line label,
width is traded for height.

Signed-off-by: Bert Timmerman <bert.timmerman@xs4all.nl>
Removed padding pixels.

Signed-off-by: Bert Timmerman <bert.timmerman@xs4all.nl>