Fixed processing multiple JPEG EXIF markers

[radarhere]

Resolves #8116

#7496 added concatenating of multiple JPEG EXIF markers.

However, the resulting image may not have the correct data in getexif(), as getexif() might be called after reading the first EXIF marker in attempt to read the DPI

Pillow/src/PIL/JpegImagePlugin.py

Lines 163 to 165 in 53e82e4

	if "dpi" not in self.info and "exif" in self.info:
	try:
	exif = self.getexif()

and getexif() is cached from that point on.

Pillow/src/PIL/Image.py

Lines 1479 to 1488 in 53e82e4

	def getexif(self) -> Exif:
	"""
	Gets EXIF data from the image.
	:returns: an :py:class:`~PIL.Image.Exif` object.
	"""
	if self._exif is None:
	self._exif = Exif()
	elif self._exif._loaded:
	return self._exif

So this PR moves the DPI reading until after all markers are processed.

---

I've updated the test image from #7496, using the following code.

>>> from PIL import Image
>>> exif = Image.Exif()
>>> exif[270] = "firstsecond"
>>> exif.tobytes()
b'Exif\x00\x00MM\x00*\x00\x00\x00\x08\x00\x01\x01\x0e\x00\x02\x00\x00\x00\x0c\x00\x00\x00\x1a\x00\x00\x00\x00firstsecond\x00'

im = Image.new("L", (1, 1))
im.save("Tests/images/multiple_exif.jpg", exif=b'Exif\x00\x00MM\x00*\x00\x00\x00\x08\x00\x01\x01\x0e\x00\x02\x00\x00\x00\x0c\x00\x00\x00\x1a\x00\x00\x00\x00first', extra=b'\xFF\xE1\x00\x0FExif\x00\x00second\x00')