video-format: Conversion from I420_10LE/BE, I420_12LE/BE, A420_10LE/BE to BGRA/RGBA creates corrupted output

Submitted by Sebastian Dröge @slomo

Link to original bug (#780189)

Description

See e.g.

gst-launch-1.0 videotestsrc ! "video/x-raw,format=I420_10LE" ! videoconvert ! "video/x-raw,format=BGRA" ! glimagesink

Same with I420_10BE, I420_12BE/LE. I422_* and Y444_* and their alpha variants work fine. Conversion to AYUV, I420, AYUV64, ARGB64 also works fine.