+ image-filters: add support for an optional function like syntax
+ image-filters

accept following syntax in agg-stack-blur:

```c++
agg-stack-blur
agg-stack-blur()  // rx = 1, ry = 1

agg-stack-blur(10) // rx = 10, ry = 1

agg-stack-blur(10,10) // rx=10,ry = 10

```