GtkNotebook: Use gdouble in coordinate calculations

GtkNotebook: Use gdouble in coordinate calculations

Otherwise rounding errors fool the "tab under coordinates" checks on
crossing events, which will be triggered close enough to the window
rectangle if the pointer moves slowly enough.

With this, the tab_prelight() function correctly figures out we've
moved the pointer outside the tab area when called in
gtk_notebook_leave_notify().

https://bugzilla.gnome.org/show_bug.cgi?id=759091