d3d11memory: Allow null GstD3D11Allocator to alloc methods
d3d11: Don't find global default allocator
We were using global default allocator already. Pass null
allocator object to *_alloc() methods then the method will
use default allocator.
d3d11memory: Allow null GstD3D11Allocator to alloc methods
Similar to gst_allocator_alloc(), use default GstD3D11Allocator
when caller passes null allocator object
d3d11: Use std::call_once()
g_once_init_enter() always takes global mutex for non-GCC build.
Use C++ once call implementation
Edited by Seungha Yang