Arnulf Rainer has been called the most important living Austrian artist. Having honored the abstract painter with an exhibition five years ago, the Albertina Museum is doing it again, this time on the occasion of Rainer’s 90th birthday. His famed crosses, overpaintings, masks, and Face Farces are here, as well as the “veil” pictures, works of iridescence that convey a Rainer oxymoron: veiling unveils. —J.V.