Add: support for emscripten (play-OpenTTD-in-the-browser)

Emscripten compiles to WASM, which can be loaded via
HTML / JavaScript. This allows you to play OpenTTD inside a
browser.

Co-authored-by: milek7 <me@milek7.pl>

src/openttd.cpp

@@ -73,6 +73,11 @@
 
 #include "safeguards.h"
 
+#ifdef __EMSCRIPTEN__
+#	include <emscripten.h>
+#	include <emscripten/html5.h>
+#endif
+
 void CallLandscapeTick();
 void IncreaseDate();
 void DoPaletteAnimations();
@@ -104,6 +109,15 @@ void CDECL usererror(const char *s, ...)
 	ShowOSErrorBox(buf, false);
 	if (VideoDriver::GetInstance() != nullptr) VideoDriver::GetInstance()->Stop();
 
+#ifdef __EMSCRIPTEN__
+	emscripten_exit_pointerlock();
+	/* In effect, the game ends here. As emscripten_set_main_loop() caused
+	 * the stack to be unwound, the code after MainLoop() in
+	 * openttd_main() is never executed. */
+	EM_ASM(if (window["openttd_syncfs"]) openttd_syncfs());
+	EM_ASM(if (window["openttd_abort"]) openttd_abort());
+#endif
+
 	exit(1);
 }