I've gone in circles a few times with how to think about image buffer management because I also support DirectDraw, which is designed to be backed by accelerated graphics, with operations like scaling bitblit. (Currently the Theseus implementation uses a shared "Surface" type as the backing store for both GDI Windows and DirectX Surfaces.)
It's a bit complicated by a few things. (1) DirectX surfaces can be "locked" to access as pixel buffers, so any accelerated surface indirection I guess would need to be able to copy pixels back down into emulator memory. Which I guess I could just implement. (2) There's a bunch of different modes for operations like bitblit like setting a color key for transparency that I can't implement with the canvas API, so I think I'd need to use GL shaders if I want acceleration, not just canvas.