wasapisink: fix a rounding error when calculating the buffer frame count

The calculation for the frame count in the non-aligned case resulted in
a one too low buffer frame count.

This resulted in:
1) exclusive mode not working as the frame count has to match
   exactly there.
2) Buffer underruns in shared mode as the current write() code doesn't
   handle catching up to low buffer levels (fixed in the next commit)

To fix just use the wasapi API to get the buffer size which will always
be correct.

1 file changed