hints: Make SDL_VIDEODRIVER and SDL_AUDIODRIVER formal hints.

They were just environment variables before.

Fixes #5528.

src/video/SDL_video.c

@@ -425,7 +425,7 @@ SDL_VideoInit(const char *driver_name)
     i = index = 0;
     video = NULL;
     if (driver_name == NULL) {
-        driver_name = SDL_getenv("SDL_VIDEODRIVER");
+        driver_name = SDL_GetHint(SDL_HINT_VIDEODRIVER);
     }
     if (driver_name != NULL && *driver_name != 0) {
         const char *driver_attempt = driver_name;

src/video/dummy/SDL_nullvideo.c

@@ -46,6 +46,7 @@
 #include "SDL_nullvideo.h"
 #include "SDL_nullevents_c.h"
 #include "SDL_nullframebuffer_c.h"
+#include "SDL_hints.h"
 
 #define DUMMYVID_DRIVER_NAME "dummy"
 
@@ -59,7 +60,7 @@ static void DUMMY_VideoQuit(_THIS);
 static int
 DUMMY_Available(void)
 {
-    const char *envr = SDL_getenv("SDL_VIDEODRIVER");
+    const char *envr = SDL_GetHint(SDL_HINT_VIDEODRIVER);
     if ((envr) && (SDL_strcmp(envr, DUMMYVID_DRIVER_NAME) == 0)) {
         return (1);
     }