C++ tweeners

 Part of the libclaw library.
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 Tween is a C++ library providing an implementation for tweening.
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 This library allows one to interpolate the intermediate values between
 two given values during a time interval. This practice is well used in
 animation and is widely popular in the context of ActionScript/Flash
 games and websites. The goal of this library is to provide similar
 functionalities to C++ programs.