There are lots of different anaglyph glasses out there and there may be even more creative uses for stereoscopic post-processing shaders.

This is pretty much a step backwards in our code. We used to use attributes in our PP shader system a long time ago but we changed it to attributeless
for code simplicity and cleanliness. This reimplements the attribute code path as an optional path to take in the case your system doesn't work with
attributeless rendering. In this case the only shipping drivers that we can know for sure supports attributeless rendering is the Nexus 5's v95 driver
that is included in the Android 5.0 image.

I hadn't planned on implementing a work around to get post processing working in these cases, but due to us force enabling the PP shader system at all
times it sort of went up on the priority list. We can't be having a supported platform black screening at all times can we?

OGL: force enable postprocessing

This is good hygiene, and also happens to be required to build Dolphin
using Clang modules.

(Under this setup, each header file becomes a module, and each #include
is automatically translated to a module import.  Recursive includes
still leak through (by default), but modules are compiled independently,
and can't depend on defines or types having previously been set up.  The
main reason to retrofit it onto Dolphin is compilation performance - no
more textual includes whatsoever, rather than putting a few blessed
common headers into a PCH.  Unfortunately, I found multiple Clang bugs
while trying to build Dolphin this way, so it's not ready yet, but I can
start with this prerequisite.)

Let's OpenGL's PostProcessing namespace be changed to a class inheriting from VideoCommon's PostProcessing class.