[FFmpeg-soc] AAC: Rewrite windowing code

[Robert Swain]

Hello,

Loren has suggested that the AAC windowing code be rewritten such that
each frame outputs samples starting with the midpoint of last MDCT
block in the previous frame, and ending with the midpoint of last MDCT
block in the current frame. This makes sense as far as MDCT codecs go
and should simplify the code a bit as I transition to using
imdct_half() and vector_fmul_window().

Michael: What do you think about this?

Anyone: How should the valid but 'meaningless' transitions (some form
of short stop to long start or vice versa) be handled?

Rob