feGaussianBlur should be pass-through if stdDeviation is negative or zero
As part of #737 (closed), I discovered that feGaussianBlur
is not handling negative or zero stdDeviation
correctly: https://drafts.fxtf.org/filter-effects/#element-attrdef-fegaussianblur-stddeviation
Currently negative or zero is taken as an error, and the filter chain is invalidated. It should work as a pass-through filter in that case.