Experimental addition to the vorbisfile API that required a few

Experimental addition to the vorbisfile API that required a few
modifications elsewhere:

added 'ov_crosslap()' such that decode of a second clip can be primed
witht he MDCT overlap of a previous clip; this entirely eliminates any
click on sample boundaries that should otherwise match, but have a
small step error due to encoding being lossy.  It will also smooth
transitions in general purpose loops.  More detailed docs to come
after more testing.

Monty

svn path=/trunk/vorbis/; revision=4387