Fix 'window-text-pixel-size' when there are leading/trailing spaces

Fix 'window-text-pixel-size' when there are leading/trailing spaces

First, scan to find the first non-whitespace character and then
backtrack to find the beginning of the line.  The previous
algorithm always started on the non-whitespace character during
the backtrack, causing it to stop immediately and not actually
find the beginning of the line.  The same applies to the end of
line calculation.
* src/xdisp.c: (Fwindow_text_pixel_size): Fix off by one error.
(Bug#45748)

* test/src/xdisp-tests.el (xdisp-tests--window-text-pixel-size)
(xdisp-tests--window-text-pixel-size-leading-space)
(xdisp-tests--window-text-pixel-size-trailing-space): New tests.