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 (closed)

PS: I don't have hardware to test vpdau and won´t have until at least the middle of February.