The SubtitleLayout is now properly aligned with the surface.
This means the subtitles remain on top of the video in portrait
mode, rather than being huge and below it.
This makes it so that it's no longer necessary to specify the number
of downstream renderers to HlsSampleSource, FrameworkSampleSource and
ExtractorSampleSource, by forcing the downstream renderers to register
with the SampleSource instances in their constructors. This eliminates
a common source of subtle client bugs where the passed value is incorrect.
