ATLANTA — I remember hearing in 2013 that George W. Bush had taken up painting. It sounded like a surprisingly artsy, but pleasant hobby for a man who had finished two terms in the White House and was finally done with politics after decades in the game. Winston Churchill had been a painter, too, and a good one. While Churchill’s art often depicted European landscapes and friends’ estates, a new exhibit of Bush’s paintings at the Atlanta History Center is a not-at-all subtle call for comprehensive immigration reform, something he tried and failed to enact during his second term as president. T…