[Bug 754826] memory: add remap operation

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=754826

--- Comment #9 from Matthew Waters <ystreet00 at gmail.com> ---
Basically there are issues with inconsistent map states between the memory and
the map info when mapping a memory multiple times.  My original attempt at
remapping only allowed there to be one mapping outstanding.  That fails with
GstVideoFrame in the normal sysmem case where a single memory holds multiple
planes.  gst_video_frame_map will gst_video_meta_map that same memory multiple
times (one for each plane).  If one lifts the 'only one outstanding mapping'
restriction one encounters other issues such as, attempting to remap the video
frame will succeed with the first memory but fail with any subsequent remaps
(because the map flag/s has already been removed).  If we don't revert the
remap, then the memory will fail unmapping because again, the flags in the map
info aren't on the memory anymore.

I essentially came to the conclusion that remapping video frames was an
impossible endeavour with the current design using GstMemory, GstVideoFrame and
GstVideoMeta.  Remapping individual memories is however very much a
possibility.

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