rtmp: Support AMF_DATA_TYPE_MIXEDARRAY

And fix the AMF_DATA_TYPE_ARRAY parsing while at it.

A MIXEDARRAY type, as the ARRAY, store the number of elements in
an uint32 before the list. The ARRAY is strict and does not have
an OBJECT terminator, MIXEDARRAY behaves like an OBJECT type and
a different than stated number of element can be present.
rtmp: Refactor get_packet
rtmp: Rewrite embedded flv handling

Use update_offset() as done for rtmp audio, video and notifications and
read update and write the fields instead of replacing them in the rtmp
packet and then memcpying it to the output buffer.
rtmp: Replace a magic number with a macro

11 is the RTMP header size.
flv: Make onTextData parsing robust

Certain streaming servers do not preserve the order of the fields.