SDL change to SDL_GLContext causes SDL3 backend build to fail

[adrian-purser]

A commit to SDL, "Use an opaque struct typedef for SDL_GLContext - Using a struct typedef instead of a void pointer results in extra C typechecks" causes the SDL3 backend to fail since it uses a void pointer.

I added a cast to line 1056 of imgui_impl_sdl3.cpp on a fork to allow me to continue -

vd->GLContext = (SDL_GLContext)sdl_gl_context;