Iterates over the specified area of this buffer with the specified {@code processor} in descending order.
(i.e. {@code (index + length - 1)}, {@code (index + length - 2)}, ... {@code index})
@return {@code -1} if the processor iterated to or beyond the beginning of the specified area.
The last-visited index If the {@link ByteProcessor#process(byte)} returned {@code false}.
Iterates over the specified area of this buffer with the specified {@code processor} in descending order. (i.e. {@code (index + length - 1)}, {@code (index + length - 2)}, ... {@code index})