"RetinaPad" from Ryan Petrich is exactly what you're searching for.
It forces apps to landscape and you can also decide to run apps in different formats, e.g. run iPhone apps as iPad apps or iPad apps as iPhone apps and also in some other options like canvas mode etc.
There're sometimes some (optical) glitches, but that depends on the application. For example the iPhone application OmniFocus in iPad mode runs perfectly without any problem or glitch ever, but Facebook messenger in iPad mode had some problems. (I'm speaking from the Facebook messenger app a month ago, when it was an iPhone only app and before it was an universal app for iPhone and iPad)