x11-toolkits/imgui: Switch to release from docking branch
The docking branch has additional features such as Multi-Viewport and
Docking. Several ports require these features.
Improve port:
- Refresh files files/CMakeLists.txt and files/imgui-config.cmake.in.
- Fix SDL2 option.
- Add options/backends: FREETYPE_SVG GLUT SDL3 SDL3_RENDERER SDLGPU3 (some of them made by default).
- Switch do-test form example_sdl2_opengl3 to example_sdl3_sdlrenderer3.
- Partially fix warnings from portclippy.
PR: 288454 283228
Approved by: Yuri Victorovich <yuri@freebsd.org>