Add slice-min-mbs feature

Works in conjunction with slice-max-mbs and/or slice-max-size to avoid overly
small slices.
Useful with certain decoders that barf on extremely small slices.

In the case that even a slice of size slice-min-mbs exceeds slice-max-size
x264 will raise the QP until it fits.
This shouldn't cause significant quality degradation with sane choices of
slice-min-mbs and slice-max-size. For example, with slice-max-size of 2000
and slice-min-mbs of 10, each macroblock can get roughly 1600 bits.

If even this somehow can't achieve the constraints (possible with absurd
parameter choices or low qp-max), x264 will exceed the given slice-max-size
and print a warning.