Allocate enough data in buffer to hold an RTP packet with csrc_count CSRCs, a payload length of
payload_len and padding of pad_len.
buffer must be writable and all previous memory in buffer will be freed. If pad_len is >0, the
padding bit will be set. All other RTP header fields will be set to 0/FALSE.
| buffer |
a Buffer |
| payload_len |
the length of the payload |
| pad_len |
the amount of padding |
| csrc_count |
the number of CSRC entries |