Got it working too, including mouse support (but not mouse cursor support!). Code for the embedder is here: https://github.com/ardera/flutter-pi
And I, too, got the same weird texture problem; could be that it's because I didn't implement the make_resource_current callback, but that should be optional.
I don't think it's the VRAM either, cause the behaviour is the same with 64MB or 256MB VRAM. There are definitely points where all textures are loaded and visible, so maybe it's not the amount of textures that's causing this. I really hope it's just some kind of inconsistency in the raspberry pi's graphics implementation that we can hack around.
Performance is not that bad, 50-60fps with the default "flutter create" app, and I didn't even compile the engine in release mode yet.