Picture allocator changes

picture: make mastering display and content light metadata accessible when allocating pictures

As the name implies.

picture: fix default_picture_allocator() return value on failure

default_picture_allocator() was returning -1, and picture_alloc_with_edges() was then changing it to -ENOMEM. The latter also means that error values returned by a custom picture allocator defined by the user were being replaced, so fix both.