private int fill(byte[] buffer, int offset,int length) throws IOException {
		int sum = 0, len;
		while (sum<length) {
			len = is.read(buffer, offset+sum, length-sum);
			if (len<0) {
				throw new IOException("End of stream");
			}
			else sum+=len;
		}
		return sum;
	}
Reads a NAL unit in the FIFO and sends it.
If it is too big, we split it in FU-A units (RFC 3984).
src/net/majorkernelpanic/streaming/rtp/H264Packetizer.java