Do not call load() before draft() in Image.thumbnail

[radarhere]

#6528 (comment) has pointed out that by calling load() at the beginning of thumbnail(), the JPEG draft() method wouldn't perform any operations.

thumbnail starts by calling the load function. Doesn't this defeat the purpose of the draft method ?

Pillow/src/PIL/Image.py

Line 2476 in ba98fe7

self.load()

#6186 added the line because load() may change the size of the image, and so change thumbnail() size calculations.

This PR rearranges the code to suit both scenarios, so that load() is called before size calculations unless draft() returns a result.