OpenGL apps: fix transparent window issue with QOpenGLWidget
OpenGL apps: fix transparent window issue with QOpenGLWidget

As discussed in the issue #266, it seems the alpha channel is actually
used for compositing... So we need to clear the alpha channel once
rendering is done.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3