But now I'm back. I had limited time last week to do work so I have no progress to show since then. I was working on a spline interpolating filter that could be extended to be a arbitrary size scaler, but it was needlessly complicated for poor results, and I have scrapped it to work on new, more useful things.
Right now I am revising the last API addition I made (comparing the current frame to the previous frame to update only necessary pixels). It forced extra code into the backend, and with so many backends, it would get easier adoption if more of the bookkeeping was migrated to the scaler code. To simplify the addition of new plugins wishing to use this feature, the relevant code has been included in a subclass that new plugins can inherit and get all the bookkeeping for free.
Then I'll be taking a look at the OpenGL backend to implement the scalers as shaders. Hopefully with some improvements, the OpenGL backend can improve performance and quality, giving people a reason to actually use it (it is not even included in many release packages).