Implement shouldResizeBlurRadiusWithImageSize feature for blurs.

Resolves #1428.

Took the liberty of making blurRadiusAsFractionOfImageWidth/Height be mutually exclusive. Updated the header documentation as such. Not sure if this constrains any derived blur filters.

Perhaps it is worth noting in the header file what typical values are for these properties; i.e. they are on the order of ten-thousandths. Values greater than this will appear to hang a thread!