Replace SentinelDrawCallback with a define

[nspitko]

This mirrors the way ImGui does it (See also ImDrawCallback_ResetRenderState).

The primary motivation here is that it's possible to override the type of ImDrawCallback in the implementation, so any extension code needs to allow for whatever zany types the implementer may choose. (In our case, vclosure*)

With this change, the type should always be compatible, and we additionally support implementation overrides should some other extension or internal code be sitting on that sentinel value already in a way that would cause a conflict.

[nspitko]

Note that #255 needs to be resolved before this can be merged, else it'll probably introduce a crash unless the user explicitly handles it in their backend.

[thedmd]

Thank you, I reintegrated changes to 0.9 and 0.10.