ARM: update NEON mc_chroma to work with NV12 and re-enable it

Up to 10-15% faster overall.
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.

If slice-min-mbs would be violated as a result of slice-max-size, x264 will
exceed slice-max-size and print a warning.
Add slices-max feature

The H.264 spec technically has limits on the number of slices per frame. x264
normally ignores this, since most use-cases that require large numbers of
slices prefer it to. However, certain decoders may break with extremely large
numbers of slices, as can occur with some slice-max-size/mbs settings.

When set, x264 will refuse to create any slices beyond the maximum number,
even if slice-max-size/mbs requires otherwise.