trunk: measure tool always starts to measure at (0,0) (desktop coords) (rev >= 12602)

Steps to reproduce:
1) launch inkscape (default prefs, default new document)
2) switch to the measure tool ('M')
3) click and start dragging anywhere on the canvas

Expected result:
The reference point for the measured distance/angle is exactly where the drag started.

Actual result:
The reference point of the measured distance/angle is always (0,0), the lower left corner of the page.

Based on tests [*] with archived builds:
- not reproduced with r12601 and earlier revisions
- reproduced with r12602 and later revisions,
this regression was introduced in
Revision 12602: Adjusted code style of SPEllipse.
<http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/revision/12602>

[*] All tests done on OS X 10.7.5.