dynapi: Accept a comma-separated list of libraries to attempt to load.

First one to load and have the necessary symbol is the one we accept,
if any. Once we accept one, we won't try loading others.

Fixes #7613.

src/dynapi/SDL_dynapi.c

@@ -441,14 +441,28 @@ extern SDL_NORETURN void SDL_ExitProcess(int exitcode);
 
 static void SDL_InitDynamicAPILocked(void)
 {
-    const char *libname = SDL_getenv_REAL(SDL_DYNAMIC_API_ENVVAR);
+    char *libname = SDL_getenv_REAL(SDL_DYNAMIC_API_ENVVAR);
     SDL_DYNAPI_ENTRYFN entry = NULL; /* funcs from here by default. */
     SDL_bool use_internal = SDL_TRUE;
 
     if (libname) {
-        entry = (SDL_DYNAPI_ENTRYFN)get_sdlapi_entry(libname, "SDL_DYNAPI_entry");
+        while (*libname && !entry) {
+            char *ptr = libname;
+            while (SDL_TRUE) {
+                const char ch = *ptr;
+                if ((ch == ',') || (ch == '\0')) {
+                    *ptr = '\0';
+                    entry = (SDL_DYNAPI_ENTRYFN)get_sdlapi_entry(libname, "SDL_DYNAPI_entry");
+                    *ptr = ch;
+                    libname = (ch == '\0') ? ptr : (ptr + 1);
+                    break;
+                } else {
+                    ptr++;
+                }
+            }
+        }
         if (!entry) {
-            dynapi_warn("Couldn't load overriding SDL library. Please fix or remove the " SDL_DYNAMIC_API_ENVVAR " environment variable. Using the default SDL.");
+            dynapi_warn("Couldn't load an overriding SDL library. Please fix or remove the " SDL_DYNAMIC_API_ENVVAR " environment variable. Using the default SDL.");
             /* Just fill in the function pointers from this library, later. */
         }
     }