clone of clipped group filtered messes up the bounding box

[Pétery Tamás]

Make a group, clip it to a smaller visual area.

Clone the group, and add a filter to the cloned group.

The filtered area is calculated upon the unclipped parent group instead of the clipping.
Which means the renderer works not on the area in focus and mostly unnecessarily, slowing down the performance.

test file here:
https://openclipart.org/detail/269523/colour-reduction-filter-pack

(It also brings up another bug, 1652474.)

Tested on win 10 64bit Inkscape 0.92pre3 15195.

[su_v (suv-lp)]

Based on tests with archived builds (on OS X 10.7.5):
- not reproduced with Inkscape 0.48.5 r10040,
- not reproduced with lp:inkscape r12359, r12362;
- not reproduced with lp:inkscape rev <= 12620,
- reproduced with lp:inkscape rev >= 12621,
- reproduced with Inkscape 0.91 r13725,
- reproduced with lp:inkscape/0.92.x r15283,
the changes in commit 12621 seem to have exposed the reported issue:
https://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/changes/12621

12621: use geometric bbox to calculate filter effect region (Bug 1229971)
https://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/revision/12621