SCUMM: Better Player_Mac::durationToSamples

Uses the fact that 4*480*480 == 225 << 12, and the identity
(a*b)>>n == (a>>n)*b + ((a%(1<<n))*b)>>n (assuming non-overflowing math),
except the rhs uses smaller intermediate values and does not overflow(*).
Compared to the original code, this uses 1 fewer division and eliminates
the rounding error.

(*) Technical note: In some cases the right hand side of the above
identity still has possibilities of intermediate overflow, but only if
b > (1 << n), or if (b << n) overflows, neither of which are true here.
Merge pull request #345 from countingpine/patch-1

SCUMM: More precise Player_Mac::durationToSamples