fix [2071412] work around mdp 32-bits fade limitation

in the case where we fade a 32-bits surface (ie: GL_MODULATE w/ a,a,a,a + blending),
we first make a copy of the background into a RGB buffer, then we blend the 32-bits
surface as usual (without the alpha component), and finally blend the copy of
the background on top with 1-a. This uses a lot of bandwidth, but no CPU time.