Rename the frame_flags to flags. Because they are flags on the frame object it
does not need the redundant frame_ prefix.
Change the order of the metadata constructor so that the flags come before the
format and dimension arguments.
There's a new GstVideoFrameFlags enum now that contains the frame
specific flags only. GstVideoFlags does not contain the TFF/TFF/ONEFIELD
flags anymore because these are strictly frame specific.
Also add fallback to parse these fields from the GstBufferFlags in
gst_video_frame_map() if there's no GstVideoMeta attached to the buffer.
Install defaul map/unmap function on the metadata and really call the functions
instead of always calling a default implementation.
Rework some bits so that we don't have to mess with the GstMapInfo information
(adding the offset), instead pass the adjusted data pointer from the map function.
