cogl-material: Don't map the shininess value to [0,1]

cogl-material: Don't map the shininess value to [0,1]

In OpenGL the 'shininess' lighting parameter is floating point value
limited to the range 0.0→128.0. This number is used to affect the size
of the specular highlight. Cogl materials used to only accept a number
between 0.0 and 1.0 which then gets multiplied by 128.0 before sending
to GL. I think the assumption was that this is just a weird GL quirk
so we don't expose it. However the value is used as an exponent to
raise the attenuation to a power so there is no conceptual limit to
the value.

This removes the mapping and changes some of the documentation.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2222