Avoid race condition when displaying images with eog

[mconst]

First of all, thanks for maintaining Pillow all these years!

When Image.show() displays an image on Linux using eog, there's a race condition where the file can get deleted before eog opens it.

To reproduce this, you'll need a Linux system without imagemagick or graphicsmagick installed. Then you can make it happen pretty reliably, just by calling Image.show() several times:

from PIL import Image

im = Image.new('RGB', (400, 300), 'green')

for i in range(4):
    im.show()

With imagemagick installed, this correctly displays four green boxes. But if you're using eog, usually several of the images will give error messages like "No images found in file:///tmp/tmprt8btbpr.PNG" instead.

This happens because Image.show() expects its display command to run synchronously, but in fact eog runs asynchronously if another eog instance is already running -- it just asks the other instance to open the file and then immediately exits. The fix is to use eog -n instead, which forces it to always start a new instance.

[radarhere]

Good work for figuring out this solution.