app: fix too big canvas with small display.

I was testing with a 2560×1440 display, yet with ×2 scale factor, which
is kinda equivalent with 1280×720 for scaled dimensions code.
And so anyway the default size request for an empty canvas was far too
big (bigger than the work area in fact).

I'm now using small default. As for the 3:1 ratio comment, not sure
about it, since shell_width is in fact changed to -1, so it feels very
specific to someone's setup. But anyway… I left the comment.

app/display/gimpdisplayshell.c

@@ -472,9 +472,11 @@ gimp_display_shell_constructed (GObject *object)
       /* These values are arbitrary. The width is determined by the
        * menubar and the height is chosen to give a window aspect
        * ratio of roughly 3:1 (as requested by the UI team).
+       * It must be smaller than the minimal supported display
+       * dimension (1280x720 a.k.a. 720p).
        */
       image_width  = GIMP_DEFAULT_IMAGE_WIDTH;
-      image_height = GIMP_DEFAULT_IMAGE_HEIGHT / 3;
+      image_height = GIMP_DEFAULT_IMAGE_HEIGHT / 4;
     }
 
   shell->dot_for_dot = config->default_dot_for_dot;