Images: support packing edited images as OpenEXR or PNG.

This way float and multilayer images can now be packed without data loss. This
removes the as_png option and always uses the appropriate file format depending
on the image contents.

release/scripts/startup/bl_operators/image.py

@@ -132,7 +132,7 @@ class SaveDirty(Operator):
                                     " can't be re-packed" %
                                     (image.name, image.library.filepath))
                     else:
-                        image.pack(as_png=True)
+                        image.pack()
                 else:
                     filepath = bpy.path.abspath(image.filepath,
                                                 library=image.library)