New video output driver: "vmem", for direct memory access.

Sometimes our list of video output modules is still not enough for an
external libvlc application with specific requirements. For instance, an
SDL application running in full screen cannot open a new window on Linux
(this is a known libSDL limitation). This vmem video output allows it to
asynchronously render the video into an SDL_Surface, with full locking.

Resolution or pitch changes are not supported yet. I'd advise against
them however, since they often require a full chroma unload/reload
operation, which is quite expensive.

I also believe this feature should be directly integrated into libvlc,
but I don't want to be too intrusive yet.