Treat previous contents of first GIF frame as transparent

[radarhere]

Helps #2239

The first frame of the GIF from the issue (and in the GIF that I've created for this PR) has disposal method 3.

https://www.w3.org/Graphics/GIF/spec-gif89a.txt

3 - Restore to previous. The decoder is required to restore the area overwritten by the graphic with what was there prior to rendering the graphic.

At the moment, we only handle disposal method 3 for frames after the first frame.

Pillow/src/PIL/GifImagePlugin.py

Lines 276 to 280 in 75c1119

	# replace with previous contents
	if self.im:
	# only dispose the extent in this frame
	self.dispose = self._crop(self.im, self.dispose_extent)
	except (AttributeError, KeyError):

This means that disposal method 3 for the first frame is treated as disposal 0 ("No disposal specified. The decoder is not required to take any action.") or disposal 1 ("Do not dispose. The graphic is to be left in place.") instead.

So since "what was there prior to rendering the graphic" is... nothing... this PR attempts to treat the previous contents of the first frame as being transparent instead.