i_bps: change values from bytes-per-second to bits-per-second

The i_bps members of the sh_audio and dev_video structs are mostly used
for displaying the average audio and video bitrates. Keeping them in
bits-per-second avoids truncating them to bytes-per-second and changing
them back lateron.
i_bps: rename to 'bitrate' to avoid confusion

Since i_bps now contains bits/sec, rename it to reflect this change.
af_fmt2bits: change to af_fmt2bps (bytes/sample) where appropriate

In most places where af_fmt2bits is called to get the bits/sample, the
result is immediately converted to bytes/sample. Avoid this by getting
bytes/sample directly by introducing af_fmt2bps.
wasapi: simplify nAvgBytesPerSec calculation

Calculate nBlockAlign seperately to reuse in the calculation of
nAvgBytesPerSec.
va_lavc: set video bitrate if available

Set the bitrate of dec_video if it is available in avcodec.
ad_lavc: don't overwrite lavc bitrate

If the bitrate is already known in avcodec there is no need to overwrite
it again with the value from sh_audio.