use user supplied value for buffer as fallback

in my application i need a very precise hit test when grabbing a dragger, so the user does not hit an close-by dragger by accident. To accomplish that i needed to change the default maximum in Pickers.cpp to the user supplied value of buffer. In my case i use 0 as buffer value (or a value close to zero) to get a very precise hit test.

This is only a suggestion, but as the previous comment suggested the value of 4 or 5 is quite random.