audio: add double sample format

To make this easier, get rid of the direct mapping of the
AF_FORMAT_BITS_MASK bit field to number of bytes. This way we can throw
away the unused AF_FORMAT_48BIT and don't have to add ..._56BIT.

audio/fmt-conversion.c

@@ -30,6 +30,7 @@ static const struct {
     {AV_SAMPLE_FMT_S16,   AF_FORMAT_S16_NE},
     {AV_SAMPLE_FMT_S32,   AF_FORMAT_S32_NE},
     {AV_SAMPLE_FMT_FLT,   AF_FORMAT_FLOAT_NE},
+    {AV_SAMPLE_FMT_DBL,   AF_FORMAT_DOUBLE_NE},
 
     {AV_SAMPLE_FMT_NONE,  0},
 };