Use 'inline' instead of 'inline_t' in the processing lib

Just use the standard inline keyword with sufficient backwards
compatibility defines, similar to how it is done in the main decoder
and encoder libraries.
Consistently use inline instead of __inline
Unify the definition of WELS_THREAD_ERROR_CODE between libraries

This simplifies the code base, reduces the risk for mixups and
gets rid of the use of a local nonstandard typedef.
Use int32_t instead of long_t in WELS_SIGN/WELS_ABS in the processing lib

This makes them match the same macros in the main decoder/encoder
libraries. long_t (which is typedeffed to long) actually is 64
bit on 64 bit unix platforms, which might not be what was
intended.
Remove the now unused long_t typedef
Make the bool_t typedef in the processing lib match the others

There is no specific reason to use int32_t for this type.
Unify the declaration of int8_t within the processing lib with the rest

int8_t in general should to be defined as signed char, since there
are actual envrionments where a plain 'char' is unsigned.

This also reduces the differences between the typedef headers of
the different sub-libraries.