Skip processing alpha if supported by codec.
Note that if the format uses pre-multiplied alpha (common with VP6,
and recommended due to better video quality/compression)
the image will look as if alpha-blended onto a black background.
However for formats that do not use pre-multiplied alpha
there might be serious artefacts (though e.g. libswscale currently
assumes pre-multiplied alpha anyway).
Skip processing alpha if supported by codec. Note that if the format uses pre-multiplied alpha (common with VP6, and recommended due to better video quality/compression) the image will look as if alpha-blended onto a black background. However for formats that do not use pre-multiplied alpha there might be serious artefacts (though e.g. libswscale currently assumes pre-multiplied alpha anyway).
- decoding: set by user - encoding: unused