Allow setting negative attitudes in plot 3D

3D plots already supported negative attitude values (looking at the plot from
"below"), but the mouse drag interface was limited to positive values in the
0-90 degree range.  This commit changes the range to -90..90.

See also: https://racket.discourse.group/t/questions-about-plot-plot3d/1134

Author: alex-hhh

plot-gui-lib/plot/private/gui/snip3d.rkt

@@ -55,7 +55,7 @@
     (define (set-altitude!)
       (define degrees-per-pixel (/ 180 (send (get-bitmap) get-height)))
       (define dy (- mouse-y left-click-y))
-      (set! altitude (clamp (+ last-altitude (* dy degrees-per-pixel)) 0 90)))
+      (set! altitude (clamp (+ last-altitude (* dy degrees-per-pixel)) -90 90)))
 
     (define (start-update-thread animating?)
       (send this start-update-thread