Replace pyglet as the default viewer in the preview() function

[asmeurer]

Right now preview() by default uses pyglet to show the image. I think it would be better to have a more portable default. Some possibilities are the PIL show function, which opens the image in the default image viewer for the platform, or using matplotlib.imshow. Probably both should be supported and the default would try each until it finds the best one that works. We can leave the pyglet viewer there, but not as the default.

[asmeurer]

I guess the change of the default should be discussed, but we should add PIL and matplotlib as viewers regardless.

[avishrivastava11]

@asmeurer +1 for the default image viewer. Although I'm not sure about using the matplotlib.imshow because of two reasons

1. Its slower than the default image viewer, and also slower to navigate (Or could only be happening in my system; If its so, please let me know)
2. It might have compatibility issues on certain systems where they don't have a certain software installed (I might be wrong here; If so, please correct me)

But I'm unaware of the method to find the default viewer and use that. Could you help me here (as I would like to take up this issue)?

[oscarbenjamin]

I have a proof of concept for using matplotlib.figshow instead of pyglet here: #8446 (comment)

[asmeurer]

We should use the system viewer as the default. There's already code there for it (#19867), so we just need to change the default.