Stop points bleeding into pixels outside of radius
Stop points bleeding into pixels outside of radius

Put in a test to break early from the density calculations when dist>radius. Stops the contribution of a point to pixels outside of it's radius, an extreme version of this is possibility of having very visible squares in the heatmap depending on radius and pixVal calculation constants.
kwauchope/heatmap,kwauchope/heatmap,kwauchope/heatmap