Paintings by and about Chicanos today can be found on prominent display in some of the nation's top museums and can command top dollar at some of this country's most exclusive art galleries. It wasn't always so.  Chicano art evolved with a provenance quite different from most conventional contemporary works of art. It is a provenance authenticated by a legacy of social, cultural and political activism. That comes into sharp focus in a new documentary film about Mexican-American art that will air Thursday night on Los Angeles public television station KCET.