Display crisp images on hidpi screens

[Yuan Fu]

Currently images on hidpi screens are sometimes blurry. I want to fix that. The 
images are blurry when the underlying terminal reports logical pixel sizes to 
Emacs, nsterm does that, I’m not sure if pgtk does that too. 

I think the best place to resolve this is at lisp level: load the image with 2x 
the dimension and add :scale 0.5. If we want to do this rescale automatically, 
lisp needs to know the “pixel radio”: the ratio between logical pixel and 
physical pixel. For hidpi this is usually 2: one logical pixel corresponds to 2 
physical pixels. I think it would be best to add this information to 
display-monitor-attributes-list. Yamamoto san mentioned that Macport does this.

Alan has a patch that returns the pixel ratio for a given frame, I tried to 
modify it to return that of a display but with no luck. 

I think pgtk can also return the pixel ratio, I remember Harano san asking 
about returning logical or physical size. 

Can x11+gtk also report pixel ratio?

Yuan