ao: don't clip floating point formats at non-unity gain

Currently, the softvol gain control attempts to clip floating point ao
formats within -1 and +1. However, this is "optimized out" at unity gain,
where no clipping is applied. This results in inconsistent behavior when
the source audio is already out of -1 and +1 range, where a gain of 0.99
results in clipping, but not at exactly 1.

Since a big advantage of floating point audio data is that they do not
lose information through out-of-range data because the ao sink can apply
suitable negative gain to prevent clipping before converting them to
integer formats, clipping should not be performed on these data.

Fix this by removing the existing clipping behavior. It now results in
a simple multiplication, which faciliates compiler auto-vectorization
of this operation over audio data.

audio/out/ao.c

@@ -612,7 +612,7 @@ void ao_set_gain(struct ao *ao, float gain)
 
 #define MUL_GAIN_f(d, num_samples, gain)                                        \
     for (int n = 0; n < (num_samples); n++)                                     \
-        (d)[n] = MPCLAMP(((d)[n]) * (gain), -1.0, 1.0)
+        (d)[n] = (d)[n] * (gain)
 
 static void process_plane(struct ao *ao, void *data, int num_samples)
 {