Remove an unused local variable
Make Wels*Snprintf return values be non-negative

This makes sure the windows version of these functions behave
more like the posix version. The posix *snprintf returns how
much would have been written if the buffer had been large
enough, which we don't know easily in the windows versions.

This basically means that we can assume that the return value is
>= 0 now, which can simplify the calling code.
Simplify the use of return values from Wels*Snprintf

Also similarly simplify the use of the return value from
WelsStrftime and GetCodeName/GetLibName/GetVerNum.
add virtual destructor to codec interface
Merge pull request #725 from licaiguo/add-virtual-destructor-api

add virtual destructor to codec interface
refine Check AU boundary
 add decide frame type for screen content
Merge pull request #728 from ruil2/enc_type2

 add decide frame type for screen content
remove warning from decoder_core.cpp
prevent from return if slice decoding error for EC
Merge pull request #726 from licaiguo/refine-auready

refine Check AU boundary
Merge pull request #731 from huili2/ec_dec_error

prevent from return if slice decoding error for EC
Merge pull request #730 from huili2/rm_warn_dec_core

remove warning from decoder_core.cpp
Merge pull request #724 from mstorsjo/string-cleanup

Unify return values from Wels*Snprintf and simplify calling code