Parallel texture registration and rendering OpenGL

#include <SDL.h>
#include <SDL_image.h>
#include <gl.h>
#include <glu.h>
#include <cstring>
#include <thread>
#include <iostream>
#include <windows.h>
#include <wingdi.h>
#define WINDOW_WIDTH 1365
#define WINDOW_HEIGHT 704
using namespace std;

SDL_Window *screen;
SDL_GLContext ctx;
GLuint red = 0;

SDL_Surface *flipSurface(SDL_Surface *surface)
{
    int current_line, pitch;
    SDL_Surface *fliped_surface = SDL_CreateRGBSurface(SDL_SWSURFACE, surface->w,surface->h, surface->format->BitsPerPixel, surface->format->Rmask, surface->format->Gmask, surface->format->Bmask, surface->format->Amask);
    SDL_LockSurface(surface);
    SDL_LockSurface(fliped_surface);
    pitch = surface->pitch;
    for(current_line = 0; current_line < surface->h; current_line++)
        memcpy(&((unsigned char*)fliped_surface->pixels)[current_line*pitch], &((unsigned char*)surface->pixels)[(surface->h - 1  - current_line)*pitch], pitch);
    SDL_UnlockSurface(fliped_surface);
    SDL_UnlockSurface(surface);
    return fliped_surface;
}

GLuint loadTexture(const char *filename)
{
    GLuint glID;
    SDL_Surface *picture_surface, *gl_surface, *gl_fliped_surface;
    Uint32 rmask, gmask, bmask, amask;
    picture_surface = IMG_Load(filename);
    if(picture_surface == NULL)
        return 0;
    #if SDL_BYTEORDER == SDL_BIG_ENDIAN
        rmask = 0xff000000;
        gmask = 0x00ff0000;
        bmask = 0x0000ff00;
        amask = 0x000000ff;
    #else
        rmask = 0x000000ff;
        gmask = 0x0000ff00;
        bmask = 0x00ff0000;
        amask = 0xff000000;
    #endif
    SDL_PixelFormat format = *(picture_surface->format);
    format.BitsPerPixel = 32;
    format.BytesPerPixel = 4;
    format.Rmask = rmask;
    format.Gmask = gmask;
    format.Bmask = bmask;
    format.Amask = amask;
    gl_surface = SDL_ConvertSurface(picture_surface, &format, SDL_SWSURFACE);
    gl_fliped_surface = flipSurface(gl_surface);
    glGenTextures(1, &glID);
    glBindTexture(GL_TEXTURE_2D, glID);
    glTexImage2D(GL_TEXTURE_2D, 0, 4, gl_fliped_surface->w, gl_fliped_surface->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, gl_fliped_surface->pixels);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    SDL_FreeSurface(gl_fliped_surface);
    SDL_FreeSurface(gl_surface);
    SDL_FreeSurface(picture_surface);
    return glID;
}

void render(GLuint picture)
{
    glBindTexture(GL_TEXTURE_2D, picture);
    glBegin(GL_QUADS);
        glTexCoord2d(0, 0); glVertex2f(-WINDOW_WIDTH, -WINDOW_HEIGHT);
        glTexCoord2d(0, 1); glVertex2f(-WINDOW_WIDTH, WINDOW_HEIGHT);
        glTexCoord2d(1, 1); glVertex2f(WINDOW_WIDTH, WINDOW_HEIGHT);
        glTexCoord2d(1, 0); glVertex2f(WINDOW_WIDTH, -WINDOW_HEIGHT);
    glEnd();
}

void registerRed()
{
    SDL_GLContext ctxa = SDL_GL_CreateContext(screen);
    bool a = wglShareLists((HGLRC)ctx, (HGLRC)ctxa);
    cout << "a" <<a;
    red = loadTexture("red.jpg");
    wglShareLists((HGLRC)ctx, (HGLRC)ctxa);
    cout << endl << "Registered: " << red << endl << endl;
}

int main(int argc, char** argv)
{
    SDL_Init(SDL_INIT_VIDEO);
    screen = SDL_CreateWindow("My App", 100, 100, WINDOW_WIDTH, WINDOW_HEIGHT, SDL_WINDOW_OPENGL);
    SDL_Event event;
    ctx = SDL_GL_CreateContext(screen);
    bool running = true;
    glEnable(GL_TEXTURE_2D);
    GLuint blue = loadTexture("blue.jpg");
    thread registerRedThread = thread(&registerRed);
    while(running)
    {
        SDL_PollEvent(&event);
        switch(event.type)
        {
            case SDL_QUIT:
                {
                    running = false;
                    break;
                }
        }

        cout << SDL_GetTicks() << endl;
        glClear(GL_COLOR_BUFFER_BIT);

        render(blue);
        if(red != 0)
        {
            cout << "Rendering the red picture..." << endl;
            render(red);
        }

        glFlush();
        SDL_GL_SwapWindow(screen);

        SDL_Delay(1000);
    }
    registerRedThread.detach();
    SDL_Quit();
    return 0;
}