QGLContext::drawTexture() doesn't work

it seems QGLContext::drawTexture() doesn't work. I try to test the corressponding functions in QGLPixelBuffer and QGLWidget which invoke QGLContext::drawTexture() directly and they couldn't draw texture on the target device.

Please refer to the file rendertextglwidget.cpp in attached package:
120 drawTexture(QPointF(30.0f, 50.0f),cubeTuxture);
121 drawTexture(QRectF(200, 200, 64, 64), cubeTuxture);

Please refer to the attachment to see the actual result and we could see that two black rectangles instead of two textures.

How to run test:
./tst_qglwidget renderText