How to generate a cube map from 6 textures for use with a shader. Uses GLGraphics and OpenGL

CubeMap.pde

import codeanticode.glgraphics.*;
import processing.opengl.*;
import javax.media.opengl.*;
import java.nio.IntBuffer;

GL gl;
GLSLShader shader;
PGraphicsOpenGL pgl;
int[] envMapTextureID = {0};
GLTexture[] textures;
String[] textureNames = {"+x.jpg", "-x.jpg", "+y.jpg", "-y.jpg", "+z.jpg", "-z.jpg"};

void setup () {
    size(400, 400, GLConstants.GLGRAPHICS);
    hint(ENABLE_OPENGL_4X_SMOOTH);
    
    pgl = (PGraphicsOpenGL) g;
    gl = pgl.gl;
    
    shader = new GLSLShader(this, "fluxus.vert", "fluxus.frag");
    
    textures = new GLTexture[6];
    for (int i = 0; i < 6; i++) {
        GLTexture tex = new GLTexture(this, textureNames[i]);
        textures[i] = tex;
    }
        
    // init cubemap textures
    gl.glGenTextures(1, envMapTextureID, 0);
    gl.glBindTexture(GL.GL_TEXTURE_CUBE_MAP, envMapTextureID[0]);
    gl.glTexParameteri(GL.GL_TEXTURE_CUBE_MAP, GL.GL_TEXTURE_WRAP_S, GL.GL_CLAMP_TO_EDGE);
    gl.glTexParameteri(GL.GL_TEXTURE_CUBE_MAP, GL.GL_TEXTURE_WRAP_T, GL.GL_CLAMP_TO_EDGE);
    gl.glTexParameteri(GL.GL_TEXTURE_CUBE_MAP, GL.GL_TEXTURE_WRAP_R, GL.GL_CLAMP_TO_EDGE);
    gl.glTexParameteri(GL.GL_TEXTURE_CUBE_MAP, GL.GL_TEXTURE_MIN_FILTER, GL.GL_NEAREST);
    gl.glTexParameteri(GL.GL_TEXTURE_CUBE_MAP, GL.GL_TEXTURE_MAG_FILTER, GL.GL_NEAREST);
    
    for (int i = 0; i < 6; i++) {
        GLTexture tex = textures[i];
        int[] pix = new int[tex.width * tex.height];
        tex.getBuffer(pix, ARGB, GLConstants.TEX_BYTE);
        gl.glTexImage2D(GL.GL_TEXTURE_CUBE_MAP_POSITIVE_X + i, 0, GL.GL_RGBA, tex.width, tex.height, 0, GL.GL_RGBA, GL.GL_UNSIGNED_BYTE, IntBuffer.wrap(pix));
    }
}

void draw () {
    background(255);
    noStroke();
    translate(mouseX, mouseY, 0);
        shader.start();
        shader.setFloatUniform("RefractionIndex", 0.5);    
        shader.setVecUniform("SpecularColour", 1.0, 1.0, 1.0);
        shader.setFloatUniform("Roughness", 1.0);
        shader.setFloatUniform("SpecularIntensity", 0.5);        
        sphere(100);
    shader.stop();  
}