BitInput#readSlice initial state of bitsRead

[collinsmith]

6be4e48 changed a problematic test case to pass stemming from a change made to BitInput#readSlice where the slice starts with an initial state of slice.bitsRead = parent.bitsRead. Should this be the expected value and not 0 bits read?

This method was added when changes were made to the DCC codec reading different compressed streams as slices so that an assertion can be made on the number of bits were read from the stream. Need to look into this and provide documentation for this.