asf: better bitrate for variable bit rate.
The spec says the following:
Alternate Data Bitrate:
Specifies the leak rate RAlt, in bits per second, of a leaky
bucket that contains the data portion of the stream without
overflowing, excluding all ASF Data Packet overhead. The size of
the leaky bucket is specified by the value of the Alternate Buffer
Size field. This value is relevant in most scenarios where the bit
rate is not exactly constant, but it is especially useful for
streams that have highly variable bit rates. This field can
optionally be set to the same value as the Data Bitrate field.
Alternate Buffer Size:
Specifies the size BAlt, in milliseconds, of the leaky bucket used
in the Alternate Data Bitrate definition. This value is relevant
in most scenarios where the bit rate is not exactly constant, but
it is especially useful for streams that have highly variable bit
rates. This field can optionally be set to the same value as the
Buffer Size field.
Alternate Initial Buffer Fullness:
Specifies the initial fullness, in milliseconds, of the leaky
bucket used in the Alternate Data Bitrate definition. This is the
fullness of the buffer at the instant before the first bit in the
stream is dumped into the bucket. Typically, this value is set to
0. This value shall not exceed the value of the Alternate Buffer
Size field.