Add picom configuration

[Morgan Aldridge]

mlvwm is compatible with the picom X11 compositor. While one can certainly use some of the advanced features, if they like, a minimal configuration prevents screen-tearing by enabling vsync.

This is my current ~/.config/picom.conf:

#
# MLVWM picom config
#
backend = "glx";
vsync = true;
xrender_sync_fence = true;
no-ewmh-fullscreen = true;

It's be nice to include this in this repository for optional use.

I'm using the glx backend for acceleration, vsync to prevent screen-tearing, and disabling EWMH (extended window manager hints) fullscreen detection because mlvwm doesn't currently have fullscreen support.

I also have an open mlvwm issue regarding adding an option to remove baked-in window/menu shadows so that one could better utilize picom's shadows functionality.