fix the range of SAD
enable frame skip in RC 1 mode
Remove some needless casts by using the right data type
Show the right message if no output file has been specified

H264DecodeInstance expects the pointer to be a null pointer if
no file has been specified.
Merge pull request #277 from lyao2/newbranch

enable frame skip in RC 1 mode
Merge pull request #278 from mstorsjo/dec-console-param

Show the right message if no output file has been specified
Merge pull request #279 from sijchen/sad_fix2

fix the range of SAD