reference gaussian_blur implementation
radius meaning match graphicmagick, pixelmator other software
  and standard deviation from classic gaussian blur algorithm
padding not using anywhere
remove pseudo vcs
rename vars
add effective_scale to all APIs
fix tests and set default effective_scale value for gaussian filter
fix radius == 0
do sum calculation in same loop as maskData calculation
round values with initial .5
this improves performance and readabillity
Merge branch 'gaussian-refactor'