Decodes four bytes from array source
and writes the resulting bytes (up to three of them)
The source and destination arrays can be manipulated
anywhere along their length by specifying
srcOffset and destOffset.
This method does not check to make sure your arrays
are large enough to accommodate srcOffset + 4 for
the source array or destOffset + 3 for
the destination array.
This method returns the actual number of bytes that
were converted from the Base64 encoding.