Screen/OSX: Match refresh rate more lenient for Screen('Resolution').

Our current Screen('Resolution(s)') API only operates with integral
values for refresh rate in Hz, but the underlying OSX functions since
our switch to the 10.6+ SDK return fractional double values. Take
this into account by matching values with a tolerance of up to 1 Hz.

Both Windows and Linux only operate at 1 Hz granularity for setting
nominal Hz, so probably doesn't make much sense to make our API
resolve fractional values.

-> Might fix the "fail to set 120 hz refresh rate" problem experienced
   when testing latest beta on OSX with a 120 Hz video projector.

-> Recompile and test on OSX pending.