This avoids reading the image size when the dimensions fields
(image_width, image_height) do not exist, as that operation may be
expensive.
Partially reverts ea53e7c09f, that dropped
the check for the dimension fields in update_dimension_fields(), because
the post_init signal was no longer registered without dimension fields.
However, another code path to that function exists: when the
ImageFileField is save()d, the name from the storage is setattr()ed on
the field, and ImageFileDescriptor calls update_dimension_fields()
because the image size might have changed. Keep bailing out early when
dimensions are unused.
Besides, computing the image dimensions causes to close() the file,
resulting in a backward-incompatible change. The test protects against
that change.
bcacc6321a
