ffx_frameinterpolation expects currentBackBuffer and currentBackBuffer_HUDLess to be identical formats

[Nukem9]

Feature request.

Code asserts that currentBackBuffer and currentBackBuffer_HUDLess inputs are identical formats. I believe this isn't necessary as long as the two formats are within the same type group. None of the upscaling/fi shader code appears to rely on this assumption. (Possibly leftover code from raster -> compute refactoring?)

There's one caveat: the previous interpolation source texture is created using the back buffer's format and down the line CopyTextureRegion(prevInterpSource, inputHUDLess) gets called. Different types aren't guaranteed to be copyable.

[Filoppi]

Attempted fix: #56