Fix#1165
In order to use transparent views, post-processing had to be disabled
because we were not able to blit post-processed buffers with blending.
This is now fixed by simply reverting to a quad.
A side effect of this, is improved performance when a scaling blit is
needed when MSAA is active.
This also introduced new "bugs", or at least weird behaviors: the
system needs to know when blending is needed, and this has to be
based on heuristics (unless we add a new api). Currently the heuristic
is that the COLOR buffer is not discarded and the view is cleared with
an alpha value (or not cleared).
dad814e046