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