'image' element opacity works differently in group

[RazrFalcon]

Image element opacity works differently in group and on image directly.
Simple example:
<g opacity="0.5"><image/> </g>
and
<g><image opacity="0.5"/> </g>
gives us different images.

Tested on Inkscape 0.91.

[su_v (suv-lp)]

On OS X 10.7.5:
- not reproduced with Inkscape 0.48.5
- reproduced with Inkscape 0.91 r13725 and 0.91+devel r14016

Based on tests with archived builds:
- not reproduced with 0.91+devel rev <= 12597,
- reproduced with 0.91+devel rev >= 12600;
this regression was possibly introduced with the changes in r12599 (for bug #804162):
http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/changes/12600
http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/revision/12599

--
replaces comment #3 (hidden) to fix typo in revision number

[su_v (suv-lp)]

Related earlier reports (AFAICT this issue existed in 0.47 and earlier too, but at the time was fixed for the initial release of Inkscape 0.48):
* Bug #284147 Imported bitmap objects rendered with wrong <100% Master Opacity
* Bug #389130 Inkscape PNG transparency display appearance does not match export and other standard applications

The earlier fix (tracked in bug #284147) was committed in
http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/revision/9429

[Mc (mc...)]

committed in r14306 with my smallest diff up to now :

- dc.paint(_opacity);
+ dc.paint(1);

// something else is probably already in charge of computing opacity, elsewhere, so it was doubly transparent

[su_v (suv-lp)]

Fix backported to 0.91.x in rev 13808.