pipewire: update to latest master

pipewire: update to latest master

Update patches:
 * remove merged patches
 * remove the alsa algorithm correction that was needed for qemu,
   as it seems to cause severe underruns on other platforms now;
   I will recheck qemu to see if this is still an issue
 * add a workaround needed to get the audioconvert plugin running
   in merge+split mode (needed to get all the channels out instead
   of just the front left channel that we were getting before)

Update version to 0.2.91 to reflect the fact that we are now tracking
the master branch instead of the work branch.

Enable the spa audiomixer plugin which is now required to mix audio on ports

Bug-AGL: SPEC-2837

Change-Id: I2558aa5487b9c9918e077bf450230c143abf7e6c
Signed-off-by: George Kiagiadakis <george.kiagiadakis@collabora.com>