hgsave and copyodbj are broken w/ gnuplot

[Dmitri A. Sergatskov]

In recent dev tests for hgsave and copyobj are failing:

E.g.:

http://buildbot.octave.org:8010/#/builders/25/builds/1104/steps/6/logs/stdio

test plot/util/copyobj.m
***** testif HAVE_MAGICK; any (strcmp ("gnuplot",
available_graphics_toolkits ()))
 toolkit = graphics_toolkit ();
 graphics_toolkit ("gnuplot");
 unwind_protect
   h1 = figure ("visible", "off");

  <...deleted...>

   assert (alpha1, alpha2);
 unwind_protect_cleanup
   close (h1);
   close (h2);
   graphics_toolkit (toolkit);
 end_unwind_protect
!!!!! test failed
ASSERT errors for:  assert (img1,img2)

  Location  |  Observed  |  Expected  |  Reason
     .       O(900x1200x3) E(656x875x3)   Dimensions don't match

Similar for hgsave:

test plot/util/hgsave.m
***** testif HAVE_MAGICK; any (strcmp ("gnuplot",
available_graphics_toolkits ()))
 toolkit = graphics_toolkit ();
 graphics_toolkit ("gnuplot");
 unwind_protect
   h1 = figure ("visible", "off");

<...deleted...>

   close (h2);
   graphics_toolkit (toolkit);
 end_unwind_protect
!!!!! test failed
ASSERT errors for:  assert (img1,img2)

  Location  |  Observed  |  Expected  |  Reason
     .       O(900x1200x3) E(656x875x3)   Dimensions don't match


Dmitri.