encoding: take 'length' procedure

We need to be able to get the length of a color array without having to
keep track of it ad hoc.
color-array-data: accessor for underlying color data
color-array-for-each: method for iterating colors in a color-array
color-array-initialize-instance: init all values to 0
color-array-for-each: fix index arithmetic
color-array-color-set!: method to set a single color in a color array