“Ghost Variations,” a chamber opera streaming in three pay-what-you-can screenings June 4 through 6 under the auspices of Thompson Street Opera, resists more than 150 years of tropes. Here, Robert Schumann’s struggle with mental illness isn’t shunted out of sight or caricatured into something malevolent; his hallucinations are sometimes frightening, sometimes darkly humorous, but always humanizing.