Current state of DPI settings under X

[Vladimir Kudrya]

Hi all!

I would like to know the current 'proper' way of setting DPI in X (if 
there is any).
I see conflicting information on the topic, and different applications 
seem to have different source of this setting. I currently counted 3 of 
them:
   - fontconfig
   - xrdb
   - randr

Different versions of gtk2 seem to either demand or ignore dpi setting 
in xrdb.
And there is also this recent change that gives xdpyinfo the ability to 
state different DPI for different outputs simultaneously: 
https://lists.x.org/archives/xorg-devel/2017-April/053430.html

Is there any DE- and toolkit-agnostic approach for letting applications 
know the proper DPI and/or notifying them that DPI has changed?