It's not about user input, but the CPU writing video hardware registers at just the right raster position mid-frame (to recycle sprites, change the color palette, or even the resolution). Home computer emulators for systems like the C64 or the Amstrad CPC need to do this at exactly the right clock cycle, otherwise modern (demo scene) demos wouldn't render correctly.
PS: of course one could build a GPU command list to render such a video frame somehow, but I bet just building this command list is more expensive then just doing the video decode with the CPU. It would basically come down to one draw command per (emulated system) pixel in the worst case.
PS: of course one could build a GPU command list to render such a video frame somehow, but I bet just building this command list is more expensive then just doing the video decode with the CPU. It would basically come down to one draw command per (emulated system) pixel in the worst case.