SDL2 Мерцание экрана

#include <SDL.h>
#include <SDL_image.h>
#include <string>
#include <iostream>
 
SDL_Window* window = NULL;
SDL_Surface *windowSurface = NULL;
SDL_Surface *imageSurface = NULL;
SDL_Renderer* renderer = NULL;
 
const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 480;
 
using namespace std;
 
void close();
 
void close()
{
    SDL_FreeSurface(imageSurface);
    imageSurface = NULL;
    window = NULL;
    windowSurface = NULL;
    renderer = NULL;
    SDL_DestroyWindow(window);
    SDL_Quit();
}
 
int main(int argc, char* args[])
{
    SDL_Init(SDL_INIT_VIDEO);
    window = SDL_CreateWindow("sdl game", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_SHOWN);
    renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);
 
    SDL_SetRenderDrawColor(renderer, 0xFF, 0xFF, 0xFF, 0xFF);
    SDL_RenderClear(renderer);
 
    int MouseX;
    int MouseY;
 
    bool isRunning = true;
    SDL_Event e;
 
    while (isRunning)
    {
        while (SDL_PollEvent(&e) != 0)
        {
            if (e.type == SDL_QUIT)
                isRunning = false;
 
            switch (e.type)
            {
            case SDL_MOUSEMOTION:
                MouseX = e.motion.x;
                MouseY = e.motion.y;
                windowSurface = SDL_GetWindowSurface(window);
 
                imageSurface = SDL_LoadBMP("btn_blue.bmp");
                SDL_Rect SrcR;
                SrcR.x = MouseX;
                SrcR.y = MouseY;
                SrcR.w = 20;
                SrcR.h = 20;
 
                SDL_BlitSurface(imageSurface, NULL, windowSurface, &SrcR);
                //SDL_UpdateWindowSurface(window);
            }
        }
        SDL_UpdateWindowSurface(window);
        SDL_RenderPresent(renderer);
    }
    close();
 
    return 0;
}