interop: vdpau: provide context in call macro

Provide the context in which the vdpau and OpenGL functions can be found
to ensure we are using initialized pointers. In particular, it now uses
the sys before it's set to the interop->priv part, instead of
dereferencing NULL.

Regression from b42d8e9e.

Fix #26553

Co-authored-by: François Cartegnie <fcvlcdev@free.fr>