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.