Added SDL_srand(), SDL_rand(), and SDL_rand_r() (thanks @JKaniarz!)

These are simple random functions that should not be used for serious random number generation.

Fixes https://github.com/libsdl-org/SDL/issues/4968

test/testoffscreen.c

@@ -21,9 +21,6 @@
 #include <emscripten/emscripten.h>
 #endif
 
-#include <stdlib.h>
-#include <time.h>
-
 static SDL_Renderer *renderer = NULL;
 static SDL_Window *window = NULL;
 static int done = SDL_FALSE;
@@ -137,8 +134,6 @@ int main(int argc, char *argv[])
 
     SDL_RenderClear(renderer);
 
-    srand((unsigned int)time(NULL));
-
 #ifndef SDL_PLATFORM_EMSCRIPTEN
     /* Main render loop */
     frames = 0;