slicetype: reorg to make frame cost estimates re-entrant (for RC and WP)

* moves frame cost calculations into a separate class deriving from WaveFront
* frames[] array now always declared on stack for better re-entrant behavior
* re-orders functions in slicetype.cpp for clarity
* internal methods were made protected, for documentation purposes only
* fixes a few minor problems discovered during the reorg
* removes deprecated ReferencePlanes.unweightedFPelPlane

--HG--
branch : stable
slicetype: upgrade frame cost variables to uint64_t

--HG--
branch : stable