Make glfw backend optional

[ptxmac]

This allows cimgui-go to "almost" be a drop-in replacement for imgui-go

I choose to make the GLFW backend opt-in since that's what makes the most sense for my use-case, but depending on other uses it might be better to have it opt-out instead

[AllenDang]

I think we should add exclude tag rather then a include tag. Because by default, cimgui should be a runnable project with default backend, unless user wants to explicitly exclude the backend.

Otherwise all down-stream package like giu should add a build tag.

[ptxmac]

I think we should add exclude tag rather then a include tag. Because by default, cimgui should be a runnable project with default backend, unless user wants to explicitly exclude the backend.

Otherwise all down-stream package like giu should add a build tag.

yeah, fine by me - I did expect you to change it before merging 😄

[AllenDang]

@ptxmac Ok, it is changed by recent commit.