endless looping GIFs with PillowWriter (matplotlib#11789)

* add loop kwarg to PillowWriter

Adds an optional parameter to PillowWriter that will set the
number of times that a gif should loop for. Defaults to 0 meaning endless

* next_whats_new file for endless gifs

* adjust PillowWriter loop kwarg

* add docstring to PillowWriter

* ValueError if loop is not an int in PillowWriter

* remove API changes

* fix typo

* kill docstring for now

Author: t-makaro

doc/users/next_whats_new/endless_gif.rst

@@ -0,0 +1,7 @@
+:orphan:
+
+Endless Looping GIFs with PillowWriter
+--------------------------------------
+
+We acknowledge that most people want to watch a gif more than once.  Saving an animation
+as a gif with PillowWriter now produces an endless looping gif.

lib/matplotlib/animation.py

@@ -562,7 +562,7 @@ def grab_frame(self, **savefig_kwargs):
     def finish(self):
         self._frames[0].save(
             self._outfile, save_all=True, append_images=self._frames[1:],
-            duration=int(1000 / self.fps))
+            duration=int(1000 / self.fps), loop=0)
 
 
 # Base class of ffmpeg information. Has the config keys and the common set