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/testgeometry.c

@@ -12,6 +12,8 @@
 
 /* Simple program:  draw a RGB triangle, with texture  */
 
+#include <stdlib.h>
+
 #include "testutils.h"
 #include <SDL3/SDL.h>
 #include <SDL3/SDL_main.h>
@@ -21,9 +23,6 @@
 #include <emscripten/emscripten.h>
 #endif
 
-#include <stdlib.h>
-#include <time.h>
-
 static SDLTest_CommonState *state;
 static SDL_bool use_texture = SDL_FALSE;
 static SDL_Texture **sprites;
@@ -260,8 +259,6 @@ int main(int argc, char *argv[])
         }
     }
 
-    srand((unsigned int)time(NULL));
-
     /* Main render loop */
     frames = 0;
     then = SDL_GetTicks();