用这两个函数实现对表面进行锁住，可以对surface->pixels进行读写。
例子：
//
// 锁住表面
//
if (SDL_MUSTLOCK(gpScreenReal))
{
if (SDL_LockSurface(gpScreenReal) < 0)
return;
}
if (!bScaleScreen)
{
screenRealHeight -= offset;
screenRealY = offset / 2;
}
if (lpRect != NULL)
{
dstrect.x = (SHORT)((INT)(lpRect->x) * gpScreenReal->w / gpScreen->w);
dstrect.y = (SHORT)((INT)(screenRealY + lpRect->y) * screenRealHeight / gpScreen->h);
dstrect.w = (WORD)((DWORD)(lpRect->w) * gpScreenReal->w / gpScreen->w);
dstrect.h = (WORD)((DWORD)(lpRect->h) * screenRealHeight / gpScreen->h);
SDL_SoftStretch(gpScreen, (SDL_Rect *)lpRect, gpScreenReal, &dstrect);
if (SDL_MUSTLOCK(gpScreenReal))
{
SDL_UnlockSurface(gpScreenReal);
}
SDL_UpdateRect(gpScreenReal, dstrect.x, dstrect.y, dstrect.w, dstrect.h);
}