Use 3 param method for converting r,g,b into css string.

This avoids creation of temporary arrays to pass them into the util
method. Also using "arguments" is more expensive then passing in 3
parameters.

FYI, here the benchmark which shows the cost of using 'arguments':
http://jsperf.com/arguments-or-explicit-use
to be hones, I did not check. i just unrolled the loop. I suspect this does not really matter much. its in annotations code. the rounded result could be different however.