toplevel-size: Report zero bounds as infinite

toplevel-size: Report zero bounds as infinite

Empty/zero bounds are sent by the Wayland compositor if there are no
valid bounds to report, e.g. if there are no connected monitors. Report
this to GTK, which uses this to clamp calculated sizes, as INT_MAX, so
that clamping isn't done until there are actual valid bounds to clamp
to.

This fixes clients sometimes shrinking to their minimum size during
hotplugs or after having suspended the session.

(cherry picked from commit b3a3a6ceb19cdc00da0d201531cff93a059a86d3)