Make number of frames available after rendering using generator.

The APNG pipeline requires the number of frames up front, and other
converters might benefit as well.  Once the frames are rendered (and don't
get re-rendered), the number of frames is obviously fixed.

As an alternative, we could turn the generator into a list when rendering,
thus avoiding the extra attribute.  But the downside of this is that the
list might be around for considerable time, eating memory.

The example animation is changed since the previous example would generate
warnings during rendering, due to negative values in the default range of x.

src/sage/plot/animate.py

@@ -185,10 +185,15 @@ class Animation(SageObject):
         ....:            for k in srange(0,2*pi,0.3)])
         sage: a.show() # optional -- ImageMagick
 
-    Do not convert input iterator to a list::
+    Do not convert input iterator to a list, but ensure that
+    the frame count is known after rendering the frames::
 
-        sage: a = animate((x^p for p in sxrange(1,2,.1))); a
+        sage: a = animate((plot(x^p, (x,0,2)) for p in sxrange(1,2,.1))); a
         Animation with unknown number of frames
+        sage: a.png()    # long time
+        '.../'
+        sage: len(a)     # long time
+        10
         sage: a._frames
         <generator object ...
 
@@ -313,7 +318,7 @@ def __add__(self, other):
         kwds = self._combine_kwds(self._kwds, other._kwds)
 
         #Combine the frames
-        m = max(len(self._frames), len(other._frames))
+        m = max(len(self), len(other))
         frames = [a+b for a,b in zip(self._frames, other._frames)]
         frames += self._frames[m:] + other._frames[m:]
 
@@ -357,7 +362,10 @@ def __len__(self):
             sage: len(a)
             5
         """
-        return len(self._frames)
+        try:
+            return self._num_frames
+        except AttributeError:
+            return len(self._frames)
 
     def make_image(self, frame, filename, **kwds):
         r"""
@@ -428,18 +436,20 @@ def png(self, dir=None):
                 return self._png_dir
             except AttributeError:
                 pass
-            d = tmp_dir()
-        else:
-            d = dir
-        G = self._frames
-        for i, frame in enumerate(self._frames):
-            filename = '%s/%s'%(d,sage.misc.misc.pad_zeros(i,8))
+            dir = tmp_dir()
+        i = 0
+        for frame in self._frames:
+            filename = '%s/%08d.png'%(dir,i)
             try:
-                frame.save_image(filename + '.png', **self._kwds)
+                save_image = frame.save_image
             except AttributeError:
-                self.make_image(frame, filename + '.png', **self._kwds)
-        self._png_dir = d
-        return d
+                self.make_image(frame, filename, **self._kwds)
+            else:
+                save_image(filename, **self._kwds)
+            i += 1
+        self._num_frames = i
+        self._png_dir = dir
+        return dir
 
     def graphics_array(self, ncols=3):
         r"""