Hide most of symbols of GstD3D11Memory object.
GstD3D11Memory is one of primary resource for imcoming d3d11 library
and it's expected to be a extensible feature.
Hiding implementation detail would be helpful for later use case.
Summary of this commit:
* Now all native Direct3D11 resources are private of GstD3D11Memory.
To access native resources, getter methods need to be used
or generic map (e.g., gst_memory_map) API should be called
apart from some exceptional case such as d3d11decoder case.
* Various helper methods are added for GstBuffer related operations
and in order to remove duplicated code.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1892>
Add new video composition element which is equivalent to compositor
and glvideomixer elements. When d3d11 decoder elements are used,
d3d11compositor can do efficient graphics memory handling
(zero copying or at least copying memory on GPU memory space).
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1323>
