Add support for rgba visuals, allowing for transparency
This PR adds a simple check for support for a RGBA visual, applying it if possible. With this, GTK themes are able to use transparency in gitg, as seen here: 2020-08-28_16-15-19
This PR adds a simple check for support for a RGBA visual, applying it if possible. With this, GTK themes are able to use transparency in gitg, as seen here: 2020-08-28_16-15-19