libtorc-video: Another buffer release fix.
libtorc-audio: Exit immediately if requested while waiting for scene
change

- and minor whitespace/typo cleanup
libtorc-audio: Fix subtitle stream flushing.
libtorc-core: Extend TorcBuffer for customisation of input buffer.

- in summary this allows a buffer sub-class to specify a required input
format (e.g. mpegts) but still require that the decoder creates an input
buffer as well.

Allows, for example, opening bluray discs without requiring a probe and
all of the associated issues that creates with state etc.
libtorc-video: Extend and fix TorcVideoOverlay

- adds support for bluray ARGB overlays.
- reverts to explicit constructors for each overlay type.

As commented, TorcVideoOverlayItem probably needs a rethink...s