Use disposal settings from previous frame in APNG

[radarhere]

Helps the APNG part of #5032. Alternative to #5071

Resolves #4657

At the moment, self.dispose_op and self.dispose_extent are set in _seek,

Pillow/src/PIL/PngImagePlugin.py

Lines 854 to 856 in ce3d80e

	self.dispose_op = self.info.get("disposal")
	self.blend_op = self.info.get("blend")
	self.dispose_extent = self.info.get("bbox")

and then used in _load_end

Pillow/src/PIL/PngImagePlugin.py

Lines 946 to 953 in ce3d80e

	if self.dispose_op == APNG_DISPOSE_OP_PREVIOUS:
	dispose = self._prev_im.copy()
	dispose = self._crop(dispose, self.dispose_extent)
	elif self.dispose_op == APNG_DISPOSE_OP_BACKGROUND:
	dispose = Image.core.fill(self.im.mode, self.size)
	dispose = self._crop(dispose, self.dispose_extent)
	else:
	dispose = None

This should not be the case, as the specification states that

dispose_op specifies how the output buffer should be changed at the end of the delay (before rendering the next frame).

It controls how the contents of the current frame should be disposed when moving on to the next frame. It is not something to be immediately applied.

This PR corrects that.