def capture
      Gl.glBindTexture Gl::GL_TEXTURE_2D, id
      Gl.glCopyTexImage2D Gl::GL_TEXTURE_2D, 0, Gl::GL_RGBA8, 0, 0, width, height, 0
      self
    end
def capture
      Gl.glBindTexture Gl::GL_TEXTURE_2D, id
      Gl.glCopyTexImage2D Gl::GL_TEXTURE_2D, 0, Gl::GL_RGBA8, 0, 0, width, height, 0
      self
    end
Copy the window contents into the buffer.