rtp: don't use memblocks for non-audio data

rtp: don't use memblocks for non-audio data

pa_memblockq_push() expects all memchunks to be aligned to PCM frame
boundaries, and that means both the index and length fields of
pa_memchunk. pa_rtp_recv(), however, used a memblock for storing both
the RTP packet metadata and the actual audio data. The metadata was
"removed" from the audio data by setting the memchunk index
appropriately, so the metadata stayed in the memblock, but it was not
played back. The metadata length is not necessarily divisible by the PCM
frame size, which caused pa_memblock_push() to crash in an assertion
with some sample specs, because the memchunk index was not properly
aligned. In my tests the metadata length was 12, so it was compatible
with many configurations, but eight-channel audio didn't work.

This patch adds a separate buffer for receiving the RTP packets. As a
result, an extra memcpy is needed for moving the audio data from the
receive buffer to the memblock buffer.

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=96612