There are times when the lights are low and the bodices have been gently loosened that “The Princess of Montpensier,” a rousing amalgam of ambition, moods and genre conceits, looks like one of those old-fashioned diversions in which swords clang as bosoms heave with sweet passion. The pretty bosom of the title character played by Mélanie Thierry tends to swell rather gently, but it also rides so high in her dresses that it evokes the temptations displayed in the windows of Parisian patisseries. The French director Bertrand Tavernier deploys some smart ideas in this film, a period story about wars on the battlefield and those closer to home, but there’s something a bit goatish in his attention to some female charms.