1\ Proposes msdk bufferpool to upstream.
- If upstream has accepted the proposed msdk bufferpool,
encoder can get msdk surface from the buffer directly.
- If not, encoder get msdk surface its own msdk bufferpool
and copy from upstream's frame to the surface.
2\ Replace arrays of surfaces with msdk bufferpool.
3\ In case of using VPP, there should be another msdk bufferpool
with NV12 info so that it could convert first and encode.
Calls gst_msdk_set_frame_allocator and uses video memory only on linux.
and uses system memory on Windows until d3d allocator is implemented.
https://bugzilla.gnome.org/show_bug.cgi?id=790752
Makes GstMsdkContext to be a descendant of GstObject so that
we could track the life-cycle of the session of the driver.
Also replaces MsdkContext with this one.
Keeps msdk_d3d.c alive for the future.
https://bugzilla.gnome.org/show_bug.cgi?id=790752
