Screenshot mode rectangle does not display in current view

Ubuntu 14.04, GIT commit: 76d6063

First reported in http://musescore.org/en/node/31586

1) create or open multi page score
2) navigate to page > 1 (so page 1 is not visible on screen)
3) enable screenshot mode

Result: rectangle does not appear. It is created on the first page, so you must scroll back there, then manually drag it back to the page on which you actually wish to to take the screenshot.

My proposed fix: display rectangle centered within view, wherever that happens to be. If re-enabling screenshot mode after a previous invocation, keep old rectangle only if part of it would be visible in the current view. Otherwise construct new one and center it.