[SDL2] PNG - czasami nie wczytuje obrazów

// SHADER SOURCES ( vertex and fragment )
const char * vTextureSource =
"attribute vec4 position;\n"
"attribute vec2 texcoord;\n"
"varying vec2 Texcoord;\n"
"void main()\n"
"{\n"
"gl_Position = position;\n"
"Texcoord = texcoord;\n"
"}\n";

const char * fTextureSource =
"precision mediump float;\n"
"uniform sampler2D texture;\n"
"varying vec2 Texcoord;\n"
"void main()\n"
"{\n"
"gl_FragColor = texture2D(texture, Texcoord);\n"
"}\n";

GLuint vTextureShader; // vertex shader
GLuint fTextureShader; // fragment shader
GLuint textureProgram; // program used our shaders

void initShader()
{
    vTextureShader = glCreateShader( GL_VERTEX_SHADER );
    fTextureShader = glCreateShader( GL_FRAGMENT_SHADER );
   
    glShaderSource( vTextureShader, 1, & vTextureSource, NULL );
    glShaderSource( fTextureShader, 1, & fTextureSource, NULL );
   
    glCompileShader( vTextureShader );
    glCompileShader( fTextureShader );
   
    textureProgram = glCreateProgram();
    glAttachShader( textureProgram, vTextureShader );
    glAttachShader( textureProgram, fTextureShader );
   
    glLinkProgram( textureProgram );
   
}

void loadTexture( GLuint & texture, string path )
{
    cout << "loading texture: " << path << "\n";
    SDL_Surface * img = IMG_Load( path.c_str() );
    if( img == NULL )
    {
        cout << "cant load texture\n";
        return;
    }
   
    // generating texture in opengles
    glGenTextures( 1, & texture );
    glBindTexture( GL_TEXTURE_2D, texture );
    // now set parameters of texture // texture must be repeat vertical and horizontal
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT );
   
    // now set the pixels to texture
    glTexImage2D(
    GL_TEXTURE_2D, 0, GL_RGBA, img->w, img->h,
    0, GL_RGBA, GL_UNSIGNED_BYTE,
    img->pixels );
   
    cout << "done\n";
    SDL_FreeSurface( img );
}

void drawRectTex( int screenWidth, int screenHeight, int posX, int posY, int width, int height, GLuint texture )
{
    // red, green, blue, alpha must have a [ 0 - 256 ] value
   
    // vertexes of Rect
    // v1 v2
    // v4 v3
   
    float v1x, v1y, v2x, v2y, v3x, v3y, v4x, v4y;
    v1x = float( posX ) - float( width ) / 2.f; v1y = float( posY ) + float( height ) / 2.f;
    v2x = float( posX ) + float( width ) / 2.f; v2y = float( posY ) + float( height ) / 2.f;
    v3x = float( posX ) + float( width ) / 2.f; v3y = float( posY ) - float( height ) / 2.f;
    v4x = float( posX ) - float( width ) / 2.f; v4y = float( posY ) - float( height ) / 2.f;
   
    static GLfloat vertexes[ 6 ]; // array x=[-1,1], y=[-1,1] for 3 vertices of triangle
    static GLfloat texCoords[ 6 ]; // x=[0,1], y=[0,1] for 3 vertices of triangle // coords of texture
    GLint textureUniform, position, texcoord;
   
    // and now we must use colorProgram ( with our shaders )
    glUseProgram( textureProgram );
   
    // prepare out texture
    glActiveTexture( GL_TEXTURE0 );
    glBindTexture( GL_TEXTURE_2D, texture );
    textureUniform = glGetUniformLocation( textureProgram, "texture" );
    glUniform1i( textureUniform, 0 );
   
    // draw first triangle
    vertexes[ 0 ] = v1x * 2 / float( screenWidth ); vertexes[ 1 ] = v1y * 2 / float( screenHeight );
    vertexes[ 2 ] = v2x * 2 / float( screenWidth ); vertexes[ 3 ] = v2y * 2 / float( screenHeight );
    vertexes[ 4 ] = v3x * 2 / float( screenWidth ); vertexes[ 5 ] = v3y * 2 / float( screenHeight );
    texCoords[ 0 ] = 0; texCoords[ 1 ] = 0;
    texCoords[ 2 ] = 1; texCoords[ 3 ] = 0;
    texCoords[ 4 ] = 1; texCoords[ 5 ] = 1;
   
    position = glGetAttribLocation( textureProgram, "position" );
    glEnableVertexAttribArray( position );
    glVertexAttribPointer( position, 2, GL_FLOAT, GL_FALSE, 0, vertexes );
   
    texcoord = glGetAttribLocation( textureProgram, "texcoord" );
    glEnableVertexAttribArray( texcoord );
    glVertexAttribPointer( texcoord, 2, GL_FLOAT, GL_FALSE, 0, texCoords );
   
    glDrawArrays( GL_TRIANGLES, 0, 4 );
   
    // draw second triangle
    vertexes[ 0 ] = v1x * 2 / float( screenWidth ); vertexes[ 1 ] = v1y * 2 / float( screenHeight );
    vertexes[ 2 ] = v3x * 2 / float( screenWidth ); vertexes[ 3 ] = v3y * 2 / float( screenHeight );
    vertexes[ 4 ] = v4x * 2 / float( screenWidth ); vertexes[ 5 ] = v4y * 2 / float( screenHeight );
    texCoords[ 0 ] = 0; texCoords[ 1 ] = 0;
    texCoords[ 2 ] = 1; texCoords[ 3 ] = 1;
    texCoords[ 4 ] = 0; texCoords[ 5 ] = 1;
   
    position = glGetAttribLocation( textureProgram, "position" );
    glEnableVertexAttribArray( position );
    glVertexAttribPointer( position, 2, GL_FLOAT, GL_FALSE, 0, vertexes );
    glDrawArrays( GL_TRIANGLES, 0, 4 );
   
    texcoord = glGetAttribLocation( textureProgram, "texcoord" );
    glEnableVertexAttribArray( texcoord );
    glVertexAttribPointer( texcoord, 2, GL_FLOAT, GL_FALSE, 0, texCoords );
   
    glDrawArrays( GL_TRIANGLES, 0, 4 );
   
}