lavf: add WebVTT muxer.

This revision creates a WebVTT muxer, that outputs files having the
format described in the following specification:

http://dev.w3.org/html5/webvtt/
lavfi/src_movie: Check pointer is not NULL before deref

Also do not check against empty string, the lower levels should
be able to deal with it.

Signed-off-by: Alexander Strasser <eclipse7@gmx.net>
lavfi/src_movie: Use movie_common_init instead individual wrappers

Makes it easier to understand that there is no difference in init
callback for movie and amovie. Also saves a few lines of code.

Signed-off-by: Alexander Strasser <eclipse7@gmx.net>
vaapi: return early from ff_vaapi_render_picture() without picture

Fixes an assertion when called on uninitialized frame. Spotted after
seeking in vlc.
Fixes ticket #2461.

Tested-by: Timo
Reviewed-by: Joakim Plate