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
As GtkCssNode has the visibility concept, it makes sense to mirror it in
gadgets.
Do what visibility does in widgets: Hidden gadgets can't be drawn or
allocated and request a 0x0 size.
Note that just like widgets, gadget visibility must not be changed in
size request, allocate or draw handlers.
GtkWidget::child-visible has no equivalent yet, code will have to
emulate that manually.
Empty boxes end up setting the clip to { 0, 0, 0, 0}, so warning
for a width or height of 0 triggers false positives. Instead,
initialize the clip to clearly invalid values.
A gadget is halfway between a widget and a CSS node. It's supposed to
provide the minimum convenicence around CSS nodes until we've figured
out how to integrate them with widgets.
