bug#67533: SVG images confound position pixel measurements

[JD Smith]

BTW, in case anyone is wondering where window-text-pixel-size with a negative 
pixel FROM offset is needed, Po Lu's pixel-scroll-precision-up-page is using 
precisely this approach to measure the content above window-start (so as to 
smoothly scroll up with high performance):

        (let* ((start (window-start))
               (dims (window-text-pixel-size nil (cons start (- delta))
                                             start nil nil nil t))