DropShadow filter: allow parameters in arbitrary order
DropShadow filter: allow parameters in arbitrary order

drop-shadow() accepts two sets of arguments:
* an optional color
* 2 or 3 lengths, describing dx, dy, and blur std_dev

These sets can be provided in any order, but cannot be intermixed;

`blue 2px 2px 3px` is valid, as is `2px 2px blue`.  However,
`2px blue 2px` is invalid.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg