sbh: early continue for all zero coeff group
refine CUSize related

rename to clarify.
division to shift.
restore WINXP_SUPPORT build option, workaround for CONDITION_VARIABLE on XP

This adapts x264's code for an XP-safe pthread_cond_t to make an XP-safe
CONDITION_VARIABLE (which was introduced in Windows Vista)

x265 will use native CONDITION_VARIABLE unless the WINXP_SUPPORT cmake option is
enabled. It forces _WIN32_WINNT=_WIN32_WINNT_VISTA for MinGW for this purpose.
vbv: enable row restarts when mid-frame qp adjustemets are inadequate
cmake: allow MinGW to target XP by default

This makes MinGW to use our workaround CONDITION_VARIABLE implementation but it
seems to be on average better than asking MinGW to compile for Vista.