Add an option to disable the kitty background color avoidance feature

[neurocyte]

The bg_collides_default logic is nice for rendering images to the terminal, but for other scenarios kitty's background color transparency is actually a pretty nice feature.

Can I suggest we add an option to disable background color avoidance? Ideally at runtime, perhaps even on a per-plane basis. But if that is too much to ask then just a global build option (or even just a define) with be great too.