frameencoder: fix handling of no thread pool being present

The encoder should run properly without a thread pool instance
encoder: move thread pool initialization to create(), handle pool options better

Previously WPP essentially owned the thread pool and the two configurations
were tied together (disabling WPP disabled the thread pool and vice-versa). Now
we have three features which use the thread pool and they can be enabled or
disabled independently of WPP.

After this commit, if all the pool features (WPP, PMODE, PME) are disabled then
the thread pool is not created.  If --threads 1 is specified, then no pool is
created.  When no pool is present, all pool options are disabled.

Reporting for all the pool options has been moved into the one log line:
x265 [info]: WPP streams / frame threads / pool  : 8 / 2 / 4 / pme / pmode

They didn't really belong on the 'tools' log line since they are not coding
tools like SAO or AMP.

This commit also fixes CTU stats logging in the absence of WPP
param: combine CTU size and RQT depth log lines (just saves space)

--HG--
extra : amend_source : 8c709c4e6dfa564ef46871e20eab9068aa803e23
analysis: fix for csp:444 decoder crash - CABAC context state handling
docs: try to explain behavior of related thread pool options
rc: vbv fix for scene changes

Also, some spacing nits

--HG--
extra : amend_source : 36841aa7d8f212547f93746d6cece29f0e4af0b8
analysis: remove Inter Part_NxN analysis and encode. This condition will never be hit.

Also remove check for intra slice in compressInterCUrd5_6

--HG--
extra : amend_source : 3e32459ac1ede95913a5e67ea94f5669155336b3
Backed out changeset: f231820645fe