rtmppkt: Rename the ts_delta field to ts_field

rtmppkt: Rename the ts_delta field to ts_field

Based on a suggestion by Martin Panter. This is more descriptive,
since it's the actual timestamp field from the RTMP packet,
which might or might not be a delta depending on context (in
some packets it's a delta, in some packets it's an absolute
timestamp, and in some packets it's 0xffffff to indicate that
the actual delta or absolute timestamp is transmitted separately).

Signed-off-by: Martin Storsjö <martin@martin.st>