Sluggish fade for small fade times

[koober]

Further to #621 , I find that trying to fade out in 40 milliseconds takes about a second.
Fixed (for me) by changing line 1187 in core from

              vol += (dir === 'in' ? 0.01 : -0.01);

to

              vol += (dir === 'in' ? diff/steps : -diff/steps);

so that 40 msecs is faded in 10 steps, 32 msecs can fade in 8 steps, and so on.

It doesn't seem to affect your tests for long fades.
I'm finding the fast fade useful for avoiding audio glitch when pausing a sprite.
( As an aside, can I also suggest changing line 1171;

       diff / 0.01 

to

       diff * 100 ,

and line 1176;

       steps / (4 / stepLen) 

to

       steps * stepLen * 0.25 

? )