build: Ignore private header while building docs
actor: Default paint volume does not work without allocation

We already check for needs_allocation before getting the default paint
volume, but explicit is better than implicit.
actor: Unallocated children do not contribute to the paint volume

Just like unmapped children.

Apparently, layers above Clutter allow mapped children without an
allocation, instead of unmapping them. This means we need to ignore
them when computing the paint volume.

Patch originally by: Adel Gadllah <adel.gadllah@gmail.com>
Signed-off by: Emmanuele Bassi <ebassi@gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=736682