First new feature added since the announcement, and it's an interesting one! This implements an option to render graphics at twice the internal resolution (424x240 -> 848x480).
For most scenarios involving pure 2D gameplay it's not useful. But once sprites start to scale, rotate, and skew, like the 3D floor here in Sonic CD's special stage, the difference becomes very noticeable. It's comparable to a feature seen in more recent SNES emulators that computes Mode 7's background layer transforms with higher precision, leading to a crisper picture.
I'm still working out how the change in rendering affects what framebuffers are used internally, and what's best for performance. Right now it causes color blending operations to run very slowly, but I hope to mitigate this issue soon. (Color blending is currently done in shader due to how Retro Engine's software renderer handled certain color effects.)