Add function for loading QImage directly in openGL format

Loading many jpeg images for use as openGL textures is very slow when using QImage. First the QImage uses ARGB when the image data has no alpha channel, then the call to QGLWidget::convertToGLFormat is an extra step which is also slow.
Adding a function for loading QImage directly in openGL friendly format and support RGB or LUMINANCE, if possible, would be very helpful.