* PLOTWINDOW: pick both corners even with ORTHO on
PLOTWINDOW's opposite-corner prompt used Prompt::base, so the canvas bent
the picked point onto the nearest axis with ORTHOMODE on: the second corner
landed above/beside the first, the window had no width or height, and
set_plot_window refused it. The prompt now uses Prompt::corner, the flag
ZOOM Window, RECTANG and ARRAYRECT already use to keep a rectangle's
opposite corner free of ortho. The first corner sets the base as before, so
the rubber band and direct distance entry still work.
Regression test drives the interactive machine through snap_cursor with
ORTHOMODE on, picks two non-axis-aligned corners and checks the stored
window keeps both extents; an ordinary LINE prompt still follows ortho.
Fixes#1641
* docs: document the functions touched by #1641
Nicholas Gower