Some frames made transparent by default when using picom/compton
Alacritty and Firefox are two examples of windows that gain "frame transparency" when picom is running with default configuration. xterm and emacs are two examples of windows that do not.

(with default frame colors)
![framenobg](https://user-images.githubusercontent.com/36526063/162595157-7a7e4a13-5f93-4595-95ee-91cae972ca35.png)
![framenobg2](https://user-images.githubusercontent.com/36526063/162595188-fafbee1c-269d-4253-a0f6-3f4b0310d4c1.png)

Transparency seems to be partially based on darkness of frame, perhaps?

I'm going to continue debugging this, but my first guess is that there's perhaps some property the frame should be setting that marks them "off limits", even if the window requests transparency (or whatever it is that firefox is doing that xterm isn't)? Or maybe something should be done when setting the background color, as the buttons (the entirety of the pixie Image it's drawn into, that is, including the portions where the window background color is drawn in) are unaffected.