Upgrade 2d density colourization to support decimal level resolution

[nicbarker]

At the moment, the 2d density colourization algorithm uses Math.floor when calculating the density of points in a square around the current point, then uses the density value to determine the colour. This means that there will be slightly more intense density colourization as x or y gets closer to zero (this could potentially be misleading for scales that exaggerate or linearize points close to zero, such as biexponential)