MSDN recommends against using the heap allocator inside DllMain(). Yet
the VLC thread variable implementation uses it.

This only affects the libav VDA implementation

When the pool is released, the pictures were destroyed only once all
references to all pooled pictures were released. Now, each picture is
destroyed individually when it is no longer referenced.

This path is about the current input thread, not about playlist
items.

Supports H.264, mp4v, H.263 and DV depending on Darwin flavor and version