vorbisenc: Separate copying audio samples from windowing

Audio samples are shifted around when copying from the frame queue so that
analysis can be done without negatively impacting calculation of the MDCT.

Window coefficients are applied to the current two overlapped windows
simultaneously instead of applying overlap for the next frame ahead of time.
This improves readability when applying windows of varying lengths.

Signed-off-by: Tyler Jones <tdjones879@gmail.com>
Reviewed-by: Rostislav Pehlivanov <atomnuker@gmail.com>
