ticket #162 : added an entry point for a potential gzip/zlib plugin or patch

Also cleaned the implementation, as compressed data is only applicable
to Base64
removed the old (now useless) parallax layer constant, and renamed the collision one

these constants were for internal usage only, so we don't need to update
the wiki ?
Finish fixing me.ImageLayer.ratio bug on Firefox & Opera

Since commit b786bc9e10965c7a6fd94307e3080278a68b2991, it was possible for
the position vector to stretch beyond the limits of the layer. This patch
uses a final modulus to clamp the limit.