[imgui] Add Freetype feature

[RT2Code]

-Add Freetype feature, which use Freetype instead of stb_truetype to build the font atlases (https://github.com/ocornut/imgui/tree/master/misc/freetype).

-Add imgui_internal.h to public includes, because it is required to use the experimental API and improve debug capabilities. It's also required to build Implot (#11920).

-Add imgui_stdlib files to allow std::string usage with Imgui text functions (ocornut/imgui#2035).

-Fix bindings feature requiring SDL1 dependency instead of SDL2 (Imgui SDL binding is using the latter).

[RT2Code]

I'd also like to suggest to remove or at least split the bindings feature. Imgui bindings are not really part of the library, and including them all in the same feature require many dependencies to be fetched, which is probably overkill in most cases. Also copying cpp files to the include folder should be avoided.

[vicroms]

Thanks for the PR!

[vicroms]

I'm not familiar with imgui, are the bindings just example code? Or what is their purpose?

I think having them as a non-default feature is good enough, some ports already have "examples" features that do the same.

[RT2Code]

These bindings are very well maintained, feature complete and the author recommend to use them by default except if a custom implementation is really needed : https://github.com/ocornut/imgui#integration

I think it's a good thing they are available as features for this port. What I dislike is the inability to pick only the required ones, it's all or nothing. Because of this, you are forced to acquire a lot of dependencies you will probably never need. It's also ugly because it isn't compiled to libs but rely instead on the manual inclusion of cpp files in your project, because some of them depends on platform specific APIs that can't be compiled everywhere.

Splitting these bindings to different features would solve these problems. The only drawback is that we would have a very long list of features :

• Allegro5
• Dx9
• Dx10
• Dx11
• Dx12
• Glfw
• Glut
• Marmalade
• Metal
• Opengl2
• Opengl3
• Osx
• Sdl
• Vulkan
• Win32

I don't know if it's acceptable for vcpkg ports to have such a long feature list, but if it is, it would be a better solution than the current one, and I'd be happy to add it.