AudioVideo: add template in ReaderProcess.

* Templating readImage in ReaderProcess: easier to maintain the
conversions (bitDepth, components...).
* Fix seg fault when indicate a wrong filePath.
* Manage seek (but not connect this to the corresponding OFX param).
* Refactoring: _idVideoStream instead of _indexVideoStream.
* Warning: currently the plugin can only read image with bitDepth=8
(waiting for new features in avTranscoder).