analysis: adding support for --rd 5/6 in compressInterCU_dist() and parallelModeAnalysis()
analysis: further work on --pmode for --rd 5/6

Still needs debugging. The results with --pmode enabled have worse compression
than without, when used with --rd 5. That should be un-possible.
analysis: checkIntraInInter_rd0_4() needs the entropy context to be initialized
analysis: invalidate slave entropy contexts, to help find bugs

this function has no effect in non-checked builds
search: further --rd-penalty 2 fixes (refs #71)
entropy: improve check strings and comments
docs: update --tu-intra-depth / --tu-inter-depth docs (refs #71)
cudata: renames (m_depth, m_trIdx) -> (m_cuDepth, m_tuDepth)

No behavior changes
cudata: setTrIdxSubParts() -> setTUDepthSubParts()
cudata: remove unused setDepthSubParts
search: trMode -> tuDepth, improve clarity
search: fixes for chroma tskip coding with placebo preset

Move the tskip decision after the special logic that deals with 4x4 luma, etc
A couple of places where tskip == 0 was implied needed to actually set the flag
search: simplify handling of TU size edge case in extractIntraResultChromaQT
entropy: nit
search: use member variable from Predict for csp

There's no need to derefernece the CU for the color space
analysis: defer broadcast sets of merge mv field data until final selection

The MC / encode functions do not need for the ME sub-parts to be set. This is
only needed by later neighbor parts
analysis: use same MV range check in merge functions
analysis: in RD 5/6 ignore redundant merge candidates
predict: cache color space dimension shifts, use in search.cpp
search: leave a helpful comment to avoid unfortunate reordering