Add support to use GraphicsMagick's "gm display" as viewer

docs/reference/ImageShow.rst

@@ -18,6 +18,7 @@ All default viewers convert the image to be shown to PNG format.
     The following viewers may be registered on Unix-based systems, if the given command is found:
 
     .. autoclass:: PIL.ImageShow.DisplayViewer
+    .. autoclass:: PIL.ImageShow.GmDisplayViewer
     .. autoclass:: PIL.ImageShow.EogViewer
     .. autoclass:: PIL.ImageShow.XVViewer
 

src/PIL/ImageShow.py

@@ -194,6 +194,15 @@ class DisplayViewer(UnixViewer):
         return command, executable
 
 
+class GmDisplayViewer(UnixViewer):
+    """The GraphicsMagick ``gm display`` command."""
+
+    def get_command_ex(self, file, **options):
+        executable = "gm"
+        command = "gm display"
+        return command, executable
+
+
 class EogViewer(UnixViewer):
     """The GNOME Image Viewer ``eog`` command."""
 
@@ -220,6 +229,8 @@ class XVViewer(UnixViewer):
 if sys.platform not in ("win32", "darwin"):  # unixoids
     if shutil.which("display"):
         register(DisplayViewer)
+    if shutil.which("gm"):
+        register(GmDisplayViewer)
     if shutil.which("eog"):
         register(EogViewer)
     if shutil.which("xv"):