Encodes up to three bytes of the array source
and writes the resulting four Base64 bytes to destination.
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 + 3 for
the source array or destOffset + 4 for
the destination array.
The actual number of significant bytes in your array is
given by numSigBytes.