Fixes integer truncation bugs (closes #359).

It is already inherited by the base class, and we don't need to import
it explicitly.

Simplify the formula, and I guess this makes the formula more
reliable.  Imagine somebody configured `buffered_before_play` larger
than 25%; then the decoder would be woken up all the time.  This
doesn't seem logical.  On the other hand, it's easy to understand that
the decoder should be woken up below 75% buffer fill.

Calculate the value only once.

Shouldn't ever happen, but who knows...

The `SliceBuffer` constructor can throw.

This adds support for the full set of MPD 0.21 filter types.

That way, each plugin can decide to implement it better.