Once exif data is parsed, do not reload unless it changes

[radarhere]

Resolves #6334

The issue features a user calling im.getexif(), modifying the exif data, and then saving to a TIFF file.

However, when saving TIFF makes sure that the exif data has been read with getexif(), which then loads the data with Exif.load_from_fp. This reloads the exif data, overriding the user's changes.

The solution is to prevent the exif data from being loaded again if it has already been parsed.

While Exif.load() checks that it is not reloading the same input, this is not done for Exif.load_from_fp(). To add that check specifically for load_from_fp would require storing the file pointer to ensure that it is the same file being read, and storing the file pointer would mean that it needs to be considered as part of the file handling lifecycle.

Instead, this PR adds a _loaded attribute. Once set, getexif() does not load the exif data anymore, but just returns the existing object. The attribute is cleared when the exif data is changed by seek operations so that the updated exif data can be loaded.

[gverm]

@radarhere Just tested this PR and it also fixes the issue.

[bigcat88]

@radarhere in my plugin pillow-heif i got a problem with new code.
there are separate exif info for each frame in HEIF, and with this changes, when changing to next frame exif thinks that it is already loaded.

        if self._exif is None:
            self._exif = Exif()
            self._exif._loaded = False
        elif self._exif._loaded:               # <--- this is always true when use ImageSequence.Iterator in Pillow 9.2
            return self._exif
        self._exif._loaded = True

should I reset in plugin self._exif._loaded this value when changing frame throw seek operation?

[radarhere]

Hi. You could set self._exif._loaded = False in the plugin, sure.

Without knowing exactly what your code does, I might alternatively suggest calling self._reload_exif(), so that if the user has previously called getexif(), that object will be updated at the same time.