Add ImGui bindings to Lua

third_party/imgui/examples/libs/emscripten/emscripten_mainloop_stub.h

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+// What does this file solves?
+// - Since Dear ImGui 1.00 we took pride that most of our examples applications had their entire
+//   main-loop inside the main() function. That's because:
+//   - It makes the examples easier to read, keeping the code sequential.
+//   - It permit the use of local variables, making it easier to try things and perform quick
+//     changes when someone needs to quickly test something (vs having to structure the example
+//     in order to pass data around). This is very important because people use those examples
+//     to craft easy-to-past repro when they want to discuss features or report issues.
+//   - It conveys at a glance that this is a no-BS framework, it won't take your main loop away from you.
+//   - It is generally nice and elegant.
+// - However, comes Emscripten... it is a wonderful and magical tech but it requires a "main loop" function.
+// - Only some of our examples would run on Emscripten. Typically the ones rendering with GL or WGPU ones.
+// - I tried to refactor those examples but felt it was problematic that other examples didn't follow the
+//   same layout. Why would the SDL+GL example be structured one way and the SGL+DX11 be structured differently?
+//   Especially as we are trying hard to convey that using a Dear ImGui backend in an *existing application*
+//   should requires only a few dozens lines of code, and this should be consistent and symmetrical for all backends.
+// - So the next logical step was to refactor all examples to follow that layout of using a "main loop" function.
+//   This worked, but it made us lose all the nice things we had...
+
+// Since only about 3 examples really need to run with Emscripten, here's our solution:
+// - Use some weird macros and capturing lambda to turn a loop in main() into a function.
+// - Hide all that crap in this file so it doesn't make our examples unusually ugly.
+//   As a stance and principle of Dear ImGui development we don't use C++ headers and we don't
+//   want to suggest to the newcomer that we would ever use C++ headers as this would affect
+//   the initial judgment of many of our target audience.
+// - Technique is based on this idea: https://github.com/ocornut/imgui/pull/2492/
+#ifdef __EMSCRIPTEN__
+#include <emscripten.h>
+#include <functional>
+static std::function<void()>            MainLoopForEmscriptenP;
+static void MainLoopForEmscripten()     { MainLoopForEmscriptenP(); }
+#define EMSCRIPTEN_MAINLOOP_BEGIN       MainLoopForEmscriptenP = [&]()
+#define EMSCRIPTEN_MAINLOOP_END         ; emscripten_set_main_loop(MainLoopForEmscripten, 0, true)
+#else
+#define EMSCRIPTEN_MAINLOOP_BEGIN
+#define EMSCRIPTEN_MAINLOOP_END
+#endif