[C++] Including `imgui-godot.h` seems to break `SubViewport` inheriting classes

[ashifolfi]

While setting up the C++ API to be used in our game's GDExtension I found a very very big flaw in the C++ API for this plugin. Including imgui-godot.h in a file that pulls in SubViewport inheriting classes will break any classes that inherit from SubViewport in C++ code, throwing errors within the GDCLASS macro.

[pkdawson]

Reproduced, thanks for reporting. The header file is a bit weird because it supports both GDExtension and modules. Those using statements weren't a great idea, I'll have to come up with a different solution.