emscripten: Let SDL hints be set by URL parameters.

Any parameters (key/value pairs after the '?' in a URL) that have a keyname
that starts with `SDL_` will be put into Emscripten's environment variable
emulation table at startup, before SDL_main runs.

This lets users set hints the same way they might set them from a shell's
command line on a desktop platform:

For example:

`https://example.com/my_sdl3_application.html?SDL_RENDER_DRIVER=software`

Fixes #10154.

CMakeLists.txt

@@ -1480,6 +1480,8 @@ elseif(EMSCRIPTEN)
   # project. Uncomment at will for verbose cross-compiling -I/../ path info.
   sdl_compile_options(PRIVATE "-Wno-warn-absolute-paths")
 
+  sdl_glob_sources("${SDL3_SOURCE_DIR}/src/core/emscripten/*.c")
+
   sdl_glob_sources("${SDL3_SOURCE_DIR}/src/main/emscripten/*.c")
   set(HAVE_SDL_MAIN_CALLBACKS TRUE)
 

include/SDL3/SDL_main.h

@@ -94,6 +94,16 @@
         /* We need to export SDL_main so it can be launched from Java */
         #define SDLMAIN_DECLSPEC    SDL_DECLSPEC
 
+    #elif defined(SDL_PLATFORM_EMSCRIPTEN)
+        /* On Emscripten, SDL provides a main function that converts URL
+           parameters that start with "SDL_" to environment variables, so
+           they can be used as SDL hints, etc.
+
+           This is 100% optional, so if you don't want this to happen, you may
+           define SDL_MAIN_HANDLED
+         */
+        #define SDL_MAIN_AVAILABLE
+
     #elif defined(SDL_PLATFORM_PSP)
         /* On PSP SDL provides a main function that sets the module info,
            activates the GPU and starts the thread required to be able to exit

src/core/emscripten/SDL_emscripten.c

@@ -0,0 +1,56 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2024 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "SDL_internal.h"
+
+#ifdef SDL_PLATFORM_EMSCRIPTEN
+
+#include <emscripten/emscripten.h>
+
+EM_JS_DEPS(sdlrunapp, "$dynCall,$stringToNewUTF8");
+
+int SDL_RunApp(int argc, char* argv[], SDL_main_func mainFunction, void * reserved)
+{
+    (void)reserved;
+
+    // Move any URL params that start with "SDL_" over to environment
+    //  variables, so the hint system can pick them up, etc, much like a user
+    //  can set them from a shell prompt on a desktop machine. Ignore all
+    //  other params, in case the app wants to use them for something.
+    MAIN_THREAD_EM_ASM({
+        var parms = new URLSearchParams(window.location.search);
+        for (const [key, value] of parms) {
+            if (key.startsWith("SDL_")) {
+                var ckey = stringToNewUTF8(key);
+                var cvalue = stringToNewUTF8(value);
+                if ((ckey != 0) && (cvalue != 0)) {
+                    //console.log("Setting SDL env var '" + key + "' to '" + value + "' ...");
+                    dynCall('iiii', $0, [ckey, cvalue, 1]);
+                }
+                _free(ckey);  // these must use free(), not SDL_free()!
+                _free(cvalue);
+            }
+        }
+    }, SDL_setenv);
+
+    return mainFunction(argc, argv);
+}
+
+#endif