Add SDL_RenderDebugTextF & SDL_RenderDebugTextV

This should make it easier to quickly put important numbers and such on
the screen without having to format them into a string manually.

examples/renderer/18-debug-text/debug-text.c

@@ -62,6 +62,9 @@ SDL_AppResult SDL_AppIterate(void *appstate)
     SDL_RenderDebugText(renderer, 14, 65, "It can be scaled.");
     SDL_SetRenderScale(renderer, 1.0f, 1.0f);
     SDL_RenderDebugText(renderer, 64, 350, "This only does ASCII chars. So this laughing emoji won't draw: 🤣");
+
+    SDL_RenderDebugTextF(renderer, 0, 0, "This program has been running for %llu seconds.", SDL_GetTicks() / 1000);
+
     SDL_RenderPresent(renderer);  /* put it all on the screen! */
 
     return SDL_APP_CONTINUE;  /* carry on with the program! */