xrender backend may create dirty trails of windows

[dccsillag]

The effect is the same as described in https://github.com/dccsillag/picom/issues/6, however, disabling window size animation (as described in that issue) does not solve the problem.

Also, this only happens in the xrender backend -- glx is fine.

It kinda looks like, for some reason, damage is not applying outside the window for some types of windows.

I've noticed that this happens in:

• qutebrowser windows
• mpv windows
• xournalpp
• brave

and this does not happen in:

• st windows
• XMonad prompt

[dccsillag]

Fixed in https://github.com/dccsillag/picom/pull/22. Once that's merged I'll close this.