[lib] Changed ImageIterator's caching criteria

- Change: `ImageIterator` now uses image file size instead of frame count.

NOTE: This change is temporary and will be reverted as soon as a Pillow version including the improvements in python-pillow/Pillow#6077 is released.

term_img/image.py

@@ -1355,11 +1355,12 @@ class ImageIterator:
           of subsequent renders) or not.
 
           - If a ``bool``, it directly sets if the frames will be cached or not.
-          - If an ``int``, caching is enabled only if the number of frames in the image
-            is less than or equal to the given number.
-          - If *repeat* equals ``1``, caching is disabled.
+          - If an ``int``, caching is enabled only if the size of the image file
+            - in bytes - is less than or equal to the given number or *image* was
+            instantiated with a PIL image.
 
     NOTE:
+        - If *repeat* equals ``1``, caching is disabled.
         - The iterator has immediate response to changes in the image
           :term:`render size` and :term:`scale`.
         - If the :term:`render size` is :ref:`unset <unset-size>`, it's automatically
@@ -1373,7 +1374,7 @@ def __init__(
         image: TermImage,
         repeat: int = -1,
         format: str = "",
-        cached: Union[bool, int] = 100,
+        cached: Union[bool, int] = 2 * 2 ** 20,
     ):
         if not isinstance(image, TermImage):
             raise TypeError(f"Invalid type for 'image' (got: {type(image).__name__})")
@@ -1397,8 +1398,13 @@ def __init__(
         self._image = image
         self._repeat = repeat
         self._format = format
+        size = (
+            0
+            if isinstance(image._source, Image.Image)
+            else os.stat(image._source).st_size
+        )
         self._cached = (
-            cached if isinstance(cached, bool) else image.n_frames <= cached
+            cached if isinstance(cached, bool) else size <= cached
         ) and repeat != 1
         self._animator = image._renderer(self._animate, alpha, fmt, check_size=False)