Fix undefined property error
Fix undefined property error

`this.options.size` in `_scalem`
ursudio/leaflet-webgl-heatmap,ursudio/webgl-heatmap-leaflet,ursudio/webgl-heatmap-leaflet,ursudio/leaflet-webgl-heatmap,ursudio/leaflet-webgl-heatmap,ursudio/webgl-heatmap-leaflet