fix level_prefix too large in residual decoding
negative data with left shift, use cast or multiplication instead
merge the calculate
Merge pull request #294 from huili2/neg_left_shift

negative data with left shift, use cast or multiplication instead
Merge pull request #292 from huili2/level_prefix

fix level_prefix too large in residual decoding
fix build fail on osx
Merge pull request #296 from ruil2/build_osx

fix build fail on osx
Merge branch 'master' of https://github.com/cisco/openh264 into develop_b