image capture: no file saved when selected rectangle is 'negative'

This issue affects the image capture function, in page-view and continuous-view modes.

MuseScore 2.1 fails to save an image file if the selected rectangle has a negative X dimension, a negative Y dimension, or both. To clarify: 'negative X dimension' refers to the situation where the initial left edge of the rectangle has been dragged rightwards past the right edge, or the initial right edge has been dragged leftwards past the left edge, so that the left and right edges have exchanged roles. (Similarly for the top and bottom edges.)

To reproduce the bug:
- load a score
- click camera icon to activate image capture
- drag the left edge of the selected (grey) rectangle to the right, past the right edge, and drop it there
- right-click over the grey rectangle
- from the pop-up menu, click on 'Save As (screenshot mode)'
- name and save the file
- expected result: file saved
- observed result: no file saved

The best solution would be to ensure that image files save correctly when a 'negative' rectangle is selected. This is apparently already the case in MuseScore 3, and is preferrable to preventing negative rectangles or introducing additional error messages.

First reported in the forum: https://musescore.org/en/node/268385#comment-816252
Video: https://drive.google.com/open?id=1c0RQ9iHcFyIlPXI4pNiZPTWDcbRUjTtW
Score on which the issue was observed: https://musescore.com/user/21654781/scores/4890472
Operating system on which the issue was observed: Windows 8.1.