Gov. Rick Perry has never been a split-the-difference kind of guy, and being a conservative in Texas means that he’s never much had to. But with his presidential campaign foundering, he has offered the Republican primary electorate a tax plan — he calls it “Cut, Balance, and Grow” — that splits the difference between the idealistic, big-ideas approach of a Newt Gingrich or a Rep. Ron Paul and the less imaginative approach of Mitt Romney, a candidate whose mind has never once been troubled by an original thought and whose every word is focus-grouped to within an inch of its …