Look up Table (LuT)
===================

For each pixel, the value is an index to search in the LuT for the real value.
When you modify a value for a pixel, you modify its index.
If the new value is not in LuT you :

    * add a new value to the LuT
    * make a new LuT copying the first one + the added value
    * throw something

Can also be handled by converting the LuT into a proper contiguous buffer.