This isn't the prettiest way to merge those functions, but it's a start.

This gives a small performance improvement for applications that are
smart enough to set the D3DPRESENTFLAG_DISCARD_DEPTHSTENCIL flag, or
to create depth stencils with Discard set to TRUE.

If the filter is paused, no additional data will be queued and 
VFW_E_TIMEOUT is legitimately returned. Don't make this a fatal error, 
but instead try again. If flushing the thing will still abort.