Multi-slice encoding support

Multi-slice encoding support
Slicing support is available through three methods (which can be mixed):
--slices sets a number of slices per frame and ensures rectangular slices (required for Blu-ray).  Overridden by either of the following options:
--slice-max-mbs sets a maximum number of macroblocks per slice.
--slice-max-size sets a maximum slice size, in bytes (includes NAL overhead).
Implement macroblock re-encoding support to allow highly accurate slice size limitation.  Might be useful for other things in the future, too.