speed up the compress filter

speed up the compress filter

Complete rewrite of the core loop of the compress filter.  The gprof
output of para_audiod (see below) indicates that CPU usage goes down
to about 40%.

new:
~~~~

Each sample counts as 0.01 seconds.
%   cumulative   self              self     total
time   seconds   seconds    calls   s/call   s/call  name

35.94      1.47     1.47     7414     0.00     0.00  compress
22.74      2.40     0.93    57319     0.00     0.00  mp3dec
16.87      3.09     0.69        1     0.69     4.04  audiod_mainloop

34.59      1.47     1.47     7414     0.00     0.00  compress
22.59      2.43     0.96        1     0.96     4.20  audiod_mainloop
17.53      3.17     0.74    61285     0.00     0.00  mp3dec

35.41      1.48     1.48     7414     0.00     0.00  compress
20.10      2.32     0.84        1     0.84     4.15  audiod_mainloop
17.82      3.06     0.74    58675     0.00     0.00  mp3dec

Average of self-seconds (3rd column):
compress		1.47
audiod_mainloop		0.83
mp3dec			0.80

0.2.11:
~~~~~~~

Each sample counts as 0.01 seconds.
%   cumulative   self              self     total
time   seconds   seconds    calls   s/call   s/call  name

56.38      3.31     3.31     7414     0.00     0.00  compress
12.93      4.08     0.76    57566     0.00     0.00  mp3dec
11.39      4.75     0.67        1     0.67     5.86  audiod_mainloop

57.59      3.38     3.38     7414     0.00     0.00  compress
14.08      4.20     0.82    63095     0.00     0.00  mp3dec
11.26      4.86     0.66        1     0.66     5.82  audiod_mainloop

57.79      4.12     4.12     7414     0.00     0.00  compress
13.62      5.08     0.97        1     0.97     7.07  audiod_mainloop
 8.08      5.66     0.57    57196     0.00     0.00  mp3dec

Average of self-seconds (3rd column):
compress 		3.60
audiod_mainloop		0.76
mp3dec			0.71