Fix for crosshair ghosting.
Fix for crosshair ghosting.

The issue is with centered or floating charts.  Firefox will report
position of DOM elements in halfs of pixels. Fixes #117.

alanponce/Flotr2,beastlike/Flotr2,jessiejea/Flotr2,jessiejea/Flotr2,beastlike/Flotr2,HumbleSoftware/Flotr2,nicgallardo/Flotr2,HumbleSoftware/Flotr2,beastlike/Flotr2,RobertoMalatesta/Flotr2,nicgallardo/Flotr2,RobertoMalatesta/Flotr2,alanponce/Flotr2,jessiejea/Flotr2,alanponce/Flotr2