Rename cabac_state.ctx to cur_ctx.
Wrap contexts of cabac_data inside cabac_data.ctx struct.
Fix broken cabac contexts in wpp.

- Fixes #84.
- The issue was caused by 241b9d6 naively copying the whole struct, which
  contains data other than just the contexts. Rather than reverting the
  change, the struct was refactored to have another struct that contained
  just the contexts.
Merge branch 'fix-wpp'