Allow transparency (rgba visual) when available and composited
This change allows GTK3 themes to make the gnome-calendar window transparent. It checks if a RGBA visual is available and if a compositor is running, and only then uses the RGBA visual, and falls back to the default screen visual otherwise.
When updating the application to GTK4, this will not be necessary anymore since they use RGBA visuals by default. (https://developer.gnome.org/gtk4/stable/gtk-migrating-3-to-4.html#id-188.8.131.52.9)
Here is a little demo video of the new, happy and exciting gnome-calendar with transparency support