SCUMM: More precise Player_Mac::durationToSamples

Untested, but the largest result in the whole calculation is `duration * _sampleRate`, so as long as it's divided by something greater than 473, the calculation is no more likely to overflow.

Note: the calculation could be made even more accurate by doing:

	(s * 473 / 480) / (4*480)
	= (s - s*7/480) / (4*480)
	~= (s - s/60 + s/480) / (4*480)

But this comes at the cost of an additional division.  It would also require more tweaking should 473 turn out to be 437.  It's perhaps no better a solution than going to 64-bit math.

EDIT: made a faster and more accurate version using bitwise operations