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
Fix UI scroll lockup.

Patch by Richard Barlow.
The attached patch fixes the UI lock up originally reported.
There are still some issues in that you can get into the state
where the layout is panning even though you're not holding the
mouse button down, but you can now exit this by clicking on
the layout.

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