Demand pixel access

[p0nce]

Suppose that before using pixels, we call a lock/unlock sequence similar to https://wiki.libsdl.org/SDL_LockSurface
Accessing pixels would only be allowed within these calls.

Then with a callback and the user pointer method, we can support any kind of lazy on-demand image.

Ideally a subimage access can be granted.
Ideally this match GPU buffer access.

[p0nce]

meh, not sure it will be a problem. This is not a CPU/GPU image abstraction, just a CPU image abstraction.