Define CpuDevice class and move CPU specific methods (color conversion) to it

[dvrogozh]

As discussed in:

• Make device interface generic #606 (review)
• Make device interface generic #606 (comment)

A follow up after #606 - need to define CpuDevice class and move SingleStreamDecoder::*OnCPU() functions to it.

CC: @scotts, @NicolasHug