st/va: set the visible image dimensions in vlVaDeriveImage

st/va: set the visible image dimensions in vlVaDeriveImage

This fixes video being rendered incorrectly.

User wants height of 360 but internally pipe_video_buffer 's height
is 368 in the test below.

Test:
  GST_GL_PLATFORM=egl gst-launch-1.0 videotestsrc ! video/x-raw, width=868, height=360, format=NV12 ! vaapipostproc ! glimagesink

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110443
Signed-off-by: Julien Isorce <jisorce@oblong.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>