vp8dec: Does not support alpha channel

Submitted by ash..@..il.com

Link to original bug (#735450)

Description

Created attachment 284508
TestVideo

The VP8 decoders do not retrieve the alpha channel from a video, I'm not exactly sure where about in the pipeline the problem lies, however when using playbin and appsink to stream a VP8 encoded video the alpha channel is simply defaulted to opaque. The VP8 codec does support an alpha channel.

Attached is a test video containing a video with an alpha channel.

Attachment 284508, "TestVideo":
StarAlpha.mkv

Version: 1.2.4