Fix discard framebuffer support in QOpenGLWidget

The enum values are different for the default framebuffer and non-default
ones. With QOpenGLWidget there is always a non-default fb bound, so the
correct way is to use GL_COLOR_ATTACHMENT0 etc. GL_COLOR_EXT and friends
are only allowed when the default framebuffer is bound, as per spec.

Change-Id: Ia8c27038dc899e44d1a95eb88adbc1cac72652b0
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>