Compton and placeholder windows

## I'm submitting a…

[x] Bug

[ ] Feature Request

[ ] Documentation Request

[ ] Other (Please describe in detail)

## Current Behavior

I am using i3 with compton and when I replace a placeholder window with a window that has transparency rules from compton they do not get applied.

I was going to attempt to assign a compton rule for the placeholder window itself to see if compton would retain the rules for the window, but using `xprop` it seems that there is no window class name or any other common value for me to use in the compton rule

## Expected Behavior

Either the window would trigger compton when it replaces the placeholder (not sure if that's possible here or if compton needs something) or the placeholder windows should have a class name so that rules in compton can be made for them.

## Environment

Output of `i3 --moreversion 2>&-`:

i3 version: 4.17.1 (2019-08-30) 

- Linux Distribution & Version: Arch Linux

- Are you using a compositor (e.g., xcompmgr or compton): compton