Allow to save tiff stacks from separate images

This is a quick solution that will allow to save tiff stacks from
separate images, e.g. from Numpy arrays.
Previously, tiff stacks could be saved only from multiframe images.
This behavior is similar to what is possible now with GIFs.

Example:

import numpy as np
from PIL import Image
a = np.ones((100,100,100), dtype=np.uint8)
imlist = []
for m in a:
    imlist.append(Image.fromarray(m))

imlist[0].save("test.tif", compression="tiff_deflate", save_all=True,
               append_images=imlist[1:])

(Should result in a 100-frame, 100x100 tiff stack.)

Signed-off-by: Leonid Bloch <leonid.bloch@esrf.fr>

PIL/TiffImagePlugin.py

@@ -1737,16 +1737,27 @@ class AppendingTiffWriter:
                 self.rewriteLastLong(offset)
 
 def _save_all(im, fp, filename):
-    if not hasattr(im, "n_frames"):
+    encoderinfo = im.encoderinfo.copy()
+    encoderconfig = im.encoderconfig
+    append_images = encoderinfo.get("append_images", [])
+    if not hasattr(im, "n_frames") and not len(append_images):
         return _save(im, fp, filename)
 
     cur_idx = im.tell()
     try:
         with AppendingTiffWriter(fp) as tf:
-            for idx in range(im.n_frames):
+            for ims in [im]+append_images:
-                im.seek(idx)
+                ims.encoderinfo = encoderinfo
-                im.load()
+                ims.encoderconfig = encoderconfig
-                _save(im, tf, filename)
+                if not hasattr(ims, "n_frames"):
+                    nfr = 1
+                else:
+                    nfr = ims.n_frames
+
+                for idx in range(nfr):
+                    ims.seek(idx)
+                    ims.load()
+                    _save(ims, tf, filename)
                     tf.newFrame()
     finally:
         im.seek(cur_idx)