feBlend-ed items wrongly rendered when grouped

[Andres Gomez (Tanty)]

Inkscape 0.48.5 r10040 for Ubuntu 14.10

Consider:
1. https://rawgit.com/tanty/design-hodgepodge/master/svg/filter/igalia.svg
2. https://rawgit.com/tanty/design-hodgepodge/master/svg/filter/igalia-feblend-no-grouped.svg
3. https://rawgit.com/tanty/design-hodgepodge/master/svg/filter/igalia-feblend-no-grouped-no-group.svg

The difference are:
1. filter including a feBlend primitive applied to the group including a set of paths
2. filter including a feBlend primitive applied individually to a set of paths
3. filter including a feBlend primitive applied to a set of paths which have been grouped

The PNG reference look for all of them should be:
https://raw.githubusercontent.com/tanty/design-hodgepodge/master/svg/filter/igalia.png

However, in the 2nd case, the result is:
2. https://rawgit.com/tanty/design-hodgepodge/master/inkscape/igalia-feblend-no-grouped.png

[su_v (suv-lp)]

On OS X 10.7.5:
- reproduced with Inkscape 0.48.5 r10040
- not reproduced with Inkscape 0.91 r13725
- not reproduced with Inkscape 0.91+devel r14425

AFAICT this known issue with the old renderer used in Inkscape <= 0.48 (rendering errors of objects with a feBlend filter inside a group) is already tracked in
* Bug #254321 “feBlend rendering problem”
* Bug #502170 “rendering artifacts with feBlend”

Proposing to link as duplicate to bug #254321 (issue is known to be fixed in current stable release 0.91).