Make sure offset is not instantiated each time the constructor is called (when using entity pooling)

I actually wanted to replace `offset` by `pos` as discussed, but the
current 'hybrid' implementation and most of all the following line
makes it impossible for now :
https://github.com/melonjs/melonJS/commit/f4974f3ab0e9d242b8ca97e87e5a6d
533418f66d#diff-1089c7ebe64469651baf69f6982fa3deR646