Include QtGui instead of glu.h (more exhaustive openGL definitions)

Dear Joker developers,

This commit addresses the following situation: on Windows, the default OpenGL header (glu.h) does not provide GL_BGR, GL_BGRA, GL_YCBCR_422_APPLE and GL_UNSIGNED_SHORT_8_8_APPLE. However, the QtGui header has been designed to handle this situation perfectly well. Using it instead of glu.h allows to remove some #ifdefs and allows one more graphic test (imageTest) to succeed on Windows.

Thanks for your attention !