OS: Linux gentoo, kernel 2.6.19-gentoo-r5

Using the software renderer of mesa, starting at id ff73c783cc47361ff0dd819c82d067b4b85870dd , 

Take the source of mesa/progs/demos/renormal.c

1. Add glClearColor(0.0,0.5,0.0,0.0); at line 32:

   GLfloat scale = 0.6 + 0.5 * sin(Phi);
   glClearColor(0.0,0.5,0.0,0.0);
   glClear( GL_COLOR_BUFFER_BIT );

2. Run renormal, the background is green, so far so good.

3. Change 
glutInitDisplayMode( GLUT_RGB | GLUT_DOUBLE );
to
glutInitDisplayMode( GLUT_RGB | GLUT_DOUBLE | GLUT_ALPHA );

4. Run renormal again, the background is black and the yellow sphere is
accumulated frame after frame, glClear() obviously does not work in this case.

Note that I added the glClearColor to make the bug more obvious. You have the
same bug without this line. The problem is really because of the GLUT_ALPHA
statement.


This example works fine with the previous id:

9f6022d0567dc1288888212d7128acc48795b306

Note that this matches the day when the depth peeling algorithm started to fail on the VTK dashboard. In VTK, depth peeling requires to have an alpha component in the framebuffer. 

http://www.vtk.org/Testing/Sites/hythloth.kitware/Linux-gcc41/20080107-0300-Nightly/Results/__Rendering_Testing_Cxx_TestOpacity.html