AVReader: refactoring

* AVReaderProcess: simplify the templated function readImage, by using
only the FileView (no tmpImage created).
* AVReaderPlugin: delete unnecessary if( _inputFile ), because it throws
exception in ensureVideoIsOpen function.
AVReader: add OFX param _paramVideoStreamIndex

OFX param to choose the index of the video stream the user wants to
transcode (0 by default).
ofxsImageEffect: getDefaultPixelDepth and getDefaultPixelComponent

* Update API of OFX plugins:
	* Rename getPixelDepth by getDefaultPixelDepth.
	* Add getDefaultPixelComponent.
	* These functions are used to return a supported pixelDepth and
a supported number of components depending on the host application (if
there is a problem with the requested one).
* Update AVReader depending on the update just above.
Common Tuttle Exception: update

* According to the OFX standard: logic error, bug, bad conversion, image
not connected, and file error, should send kOfxStatFailed intead of
kOfxStatErrValue to the host.
ofxsImageEffect: refactoring

Remove unused call of retrieveImageEffectPointer function.
ReaderPlugin: suppress warning

Unused variable.