[Libva] Horizontal stripes on low bitrates

[ValdikSS]

Hello!
I'm running libva 1.5.1 and libva-intel-driver 1.5.1 on Sandy Bridge.
If I encode any video with low (< 8 Mbit/s) bitrate I get very distorted video output with horizontal stripes in it. This is a case for both cqp and cbr encoding.
I've tried to encode it with gstreamer 1.0, avcenc and h264encode with same output result.

This is what I get with 3000 Kbit/s CBR encoding:
http://ovrload.ru/f/55938_bake-cbr-3000.mkv

It was encoded with the following gstreamer pipeline:

gst-launch-1.0 -e filesrc location=bake_op.mkv ! matroskademux ! vaapidecode ! videoconvert ! video/x-raw,format=NV12 ! vaapiencode_h264 rate-control=cbr
bitrate=3000 ! video/x-h264,stream-format=byte-stream ! h264parse ! matroskamux ! progressreport ! filesink location=bake-cbr-3000.mkv

Windows encoders like MediaCoder and handbrake produce decent video quality without horizontal stripes. Nothing different with current git master of libva and
libva-intel-driver.