Just a remark about black borders; depending on the display and emulated hardware, when using integer scaling the resulting borders can be rather small and very much acceptable, I don't know why people freak out with the idea of borders. For instance in cases you're playing on a big display it's often much more enjoyable to have a clean scaled picture with reasonable borders rather than the obligatory maximum stretching.
Also I've said it in other threads but when using a smaller display this time, having the possibility to integer-stretch outside of screen limits, resulting in the opposite (a few lines left out) is also quite enjoyable.
Of course it's very nice if alternatively using a shader(s) can still maintain visual integrity while using the entire V or H real estate, but it also means those who care about a clean picture have no choice but to use the shader(s) since an integer option is not included in base MAME to begin.
Really curious to see how all that'll evolve with BGFX.