Incorrect RTCP SR + RR parsing

[cdleonard]

Incorrect RTCP SR + RR parsing

[cdleonard]

@oborichkin

[cdleonard]

After some further digging it seems that when using consecutive PacketField those Packet sub-classes should implement a custom "extract_padding" function to determine their own length.

Made a PR for this. The test includes an adapted version of the earlier test which demonstrated the bug - it now demonstrates correct build+parse of a RTCP container SR and multiple RR

[oborichkin]

@cdleonard see #3580
This issue is fixed by adding extract_padding to SenderInfo:

    def extract_padding(self, p):
        return "", p

I honestly don't remember why I kept MR in Draft, maybe fixing conflicts will be enough for most cases.

[oborichkin]

On a second thought I remember that it was something to do with conditional fields and count_from/count_of. I'm not very happy with how they are ended up

[cdleonard]

My PR is just for this specific issue and it seems obviously correct.

Maybe it should be merged and then you can rebase any additional improvements on top?

[oborichkin]

seems reasonable