Rhode Island’s first medical marijuana dispensary has opened to patients in Providence. The Thomas C. Slater Compassion Center started selling medicinal marijuana Friday to what a spokesman called a ‘‘steady stream of patients.’’ The facility received its license from the Health Department earlier this month. The dispensary is initially selling marijuana cultivated by growers participating in the state’s medical marijuana program. But it will immediately begin growing its own product to sell. Marijuana dispensaries in Warwick and Portsmouth are expected to open in coming months.