gadget: Be careful in geometry calculations

gadget: Be careful in geometry calculations

Margins can be negative, and if we are not careful, then
content+padding+margin can end up with negative dimensions,
which can upset pixman and others. This commit ensures
that a gadget will not request or draw boxes with negative
dimensions, and get_border_allocation and get_content_allocation
will not return boxes with negative dimensions.

This fixes a crash in the paned separator drawing code that
can be reproduced by setting separator padding to 0.

https://bugzilla.gnome.org/show_bug.cgi?id=759657