Do not unconditionally fetch slang and imgui

[expipiplus1]

Also add an option to use local imgui source

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[westlicht]

what is the reason for needing the local imgui option?

[ccummingsNV]

In theory if it wasn't for the dev comment breaking CMAKE, I'd say it looked correct, but I'm also a little concerned about deciding to add the option for a 'local imgui'. If we start down that path, do we need to provide a 'local X' for any extra dependencies? Unless there's a key reason for needing it? (eg a custom build of imgui with specific features?)