videoconvert: Add support for GST_VIDEO_FLAG_PREMULTIPLIED_ALPHA
@slomo
Submitted by Sebastian Dröge Link to original bug (#666090)
Description
Currently only non-premultiplied alpha is supported. I'd add it by adding new variants of all the video formats with alpha and adding a new flag to GstVideoFormatFlags.
Adding it to GstVideoColorimetry is probably not enough because that is not negotiable currently