buffer with video meta information do not support memory split buffers

Submitted by Frediano Ziglio @fziglio

Link to original bug (#779524)

Description

Try to avoid some clipping of images and pass DRM primes to GStreamer I tried to use gst_buffer_add_video_meta_full functions. This worked well on textures (DRM primes) and frames composed by a single memory chunk but with frames (that is GstBuffer) having multiple memory chunks (GstMemory) you get memory errors.
This is due to the fact that gst_video_frame_map_id (called internally by GStreamer) assume that if there are video meta information attached there is a single chunk of memory.
This behaviour is not documented for gst_buffer_add_video_meta_full or GstVideoMeta and having multiple GstMemory is supported normally so for me is an API or documentation bug.
Looking at git master code there are no changes (either implementation or documentation) in this respect.