vaapipostproc: Does not maintain aspect ratio when scaling

Submitted by Nicolas Dufresne @ndufresne

Link to original bug (#790149)

Description

When vaapipostproc is being used (notably through vaapidecodebin), the vaapipostproc may endup scaling the video. The problem is that the scaling does not maintain the aspect ratio, which is totally unexpected default behaviour. On can illustrace this with a non scaling display sink.

gst-launch-1.0 videotestsrc ! video/x-raw,width=320,height=240 ! vaapipostproc ! ximagesink

In comparison with:

gst-launch-1.0 videotestsrc ! video/x-raw,width=320,height=240 ! videoscale ! ximagesink