Optimize CPU usage in caed alsa playback

caed alsa takes a lot of CPU. According to CPU profiling, the `AlsaPlay2Callback` is the main candidate for optimization.

This pull request provides two optimizations : 

* in Process Passthroughs, it detects when `alsa_passthrough_volume` is zero and skip the "channel".
* in Process Output Meters, it saves direct accesses in `alsa_format->card_buffer`

**Before** (about 12% of my i7 CPU) : 

```
$ google-pprof --text --lines ./cae/.libs/caed profile      
Using local file ./cae/.libs/caed.
Using local file profile.
Total: 316 samples
     245  77.5%  77.5%      283  89.6% AlsaPlay2Callback ??:?
      31   9.8%  87.3%       31   9.8% __select_nocancel /build/glibc-NmptCx/glibc-2.19/misc/../sysdeps/unix/syscall-template.S:81
      23   7.3%  94.6%       30   9.5% AlsaCapture2Callback ??:?
```

**After** (about 5.5% of my i7 CPU) : 

```
$ google-pprof --text --lines ./cae/.libs/caed profile_after 
Using local file ./cae/.libs/caed.
Using local file profile_after.
Total: 1027 samples
     405  39.4%  39.4%      775  75.5% AlsaPlay2Callback ??:?
     229  22.3%  61.7%      229  22.3% __select_nocancel /build/glibc-NmptCx/glibc-2.19/misc/../sysdeps/unix/syscall-template.S:81
     189  18.4%  80.1%      279  27.2% AlsaCapture2Callback ??:?
      20   1.9%  82.1%       26   2.5% memset /build/glibc-NmptCx/glibc-2.19/string/../sysdeps/x86_64/memset.S:94
```