Called by filter as incremental progress is made during decode.
HRESULT retVal = object.OnProgress(pBounds, bComplete);
A pointer to a variable of type RECT that specifies the rectangle where progress was made.
A BOOL that specifies TRUE if this is the final pass for this rectangle, or FALSE otherwise.
If this method succeeds, it returns S_OK. Otherwise, it returns an HRESULT error code.
Use only if IMGDECODE_EVENT_PROGRESS flag was returned in pdwEvents parameter of IImageDecodeEventSink::OnBeginDecode.