Although D3D11 video decoding is unuspported on Windows 7, the
associated APIs almost work. Where they fail is texture creation, where
we try to create D3D11_BIND_DECODER surfaces. So specifically try to
detect this situation.
One issue is that once the hwdec interop is created, the damage is done,
and it can't use another backend (because currently only 1 hwdec backend
is supported). So that's where we prevent attempts to use it.
It still can fail when trying to use d3d11va-copy (since that doesn't
require an interop backend), but at that point we don't care anymore -
dxva2(-copy) is tried before that anyway.
wm4