testbench: added stress test cases for all functions in mbdstharness.cpp
api: improve documentation of x265_picture, properly initialize pic.colorSpace
cmake: more general fix for CFLAGS breaking resource compiles

There are some versions of rc.exe that also balk at being given cl.exe flags
compress: fix gcc warning, make array const

c:/mcw/x265/source/encoder/compress.cpp:414:22: warning: variable 'bestMergePred' set but not used [-Wunused-but-set-variable]
             TComYuv* bestMergePred;
fix bug on TSKIP