Lightweight, Modular CMake Integration for ImGui

[pinwhell]

This commit introduces a minimal and clean CMake setup aimed at maintaining the simplicity of the project. The goal is to provide a foundation for modular build configurations, allowing for future additions (e.g., backend implementations, examples) without introducing bloat.

[pinwhell]

I had a few doubts, i was unsure, what is the more standard expected way when including backends

#include <backends/imgui_impl_xxxxx.h>

or directly

#include<imgui_impl_xxxxx.h>

also, what about, when installing imgui includes, is it supposed to be installed within a imgui root include folder in such a way that includes goes:

#include <imgui/imgui.h>
#include <imgui/xxxx/xxxx.h>

or direclty install imgui includes to the destination root include path, in such a way that includes looks like:

#include <imgui.h>
#include <imgui_xxxxx.h>

Note: Imgui already follows a flat looking convention when referring to includes, for example imgui consistently follows a pattern of prefixing its files with imgui_ thus suggesting that the proper decision would be to simply install the flat imgui includes that comes already namespaced in a way, i believe making the equivalence of having a imgui include folder in the installed include directory, on top of this project using imgui would benefit, by not breaking on a new convention, they will still see the flat convention as usual.

the aim is to have minimum amount of change and maximum amount of consistency.

[pinwhell]

Example:

find_package(imgui REQUIRED)
# or
# add_subdirectory(path/to/imgui)
add_executable(foo foo.cpp)
target_link_libraries(myexec 
imgui::core         # Core Agnostic Imgui Impl, ex imgui.cpp imgui_tables.cpp ... etc etc
imgui::freetype     # Imgui FreeType Implementation
...
imgui::win32        # Imgui Windows Backend
# imgui::opengl2    # Imgui Opengl2 Backend Implementation
imgui::opengl3      # Imgui Opengl3 Backend Implementation
...
)

As shown above, everything is very decoupled, concerns are very separated, in a way feels like composing your build with imgui modules!

// foo.cpp

#include <imgui.h>                  // From imgui::core
#include <imgui_impl_win32.h>       // From imgui::win32
//#include <imgui_impl_opengl2.h>   // From imgui::opengl2
#include <imgui_impl_opengl3.h>     // From imgui::opengl3
#include <imgui_freetype.h>         // From imgui::freetype

int main()
{
   Imgui:: .... (); 
}

Note: Each component shown above is automatically being linked statically! (including Freetype etc!)