It doesn't seem to be thread safe.

(cherry picked from commit ff39bd61) (edited)

edited:
- this branch doesn't have the D3D11_CreateProcessor helper
- the d3d11_device_t is stored locally rather than using a point
- the surface conversion filter didn't use a mutex at all

Signed-off-by: Steve Lhomme <robux4@ycbcr.xyz>