Only load image in ImageFile load_prepare() if palette is dirty

[radarhere]

Change

Pillow/src/PIL/ImageFile.py

Lines 452 to 454 in 3f6d442

	# create palette (optional)
	if self.mode == "P":
	Image.Image.load(self)

to only run if the palette is present and dirty, since that is what is needed to update the palette.

Pillow/src/PIL/Image.py

Line 984 in 3f6d442

if self._im is not None and self.palette and self.palette.dirty: