Bring back support for glReadPixels screenshot path

Squashed commit of the following:

commit 012d3fe41d1d6cd38a0858b59145e9a4447641fa
Author: Hashcode <hashcode0f@gmail.com>
Date:   Sun Dec 8 19:36:50 2013 +0000

    sf: Always use opengles for screen capture

    Go back to the usage of GRALLOC_USAGE_HW_TEXTURE and GRALLOC_USAGE_HW_RENDERER
    in captureScreenImplLocked regardless of useReadPixels value

    This fixes the EGL_NO_IMAGE_KHR error returned from
    eglCreateImageKHR (blank images returned from screenshot path)

    Change-Id: I62fe90a081607b9e89c67f3dcfd34c84efc89d35

commit 4866ddf98ac98d8e22a1cd6a21894bb17f274588
Author: Ricardo Cerqueira <cyanogenmod@cerqueira.org>
Date:   Thu Oct 31 03:53:39 2013 +0000

    Revert "remove support for glReadPixels screenshot path"

    This reverts commit 3ca76f416bc8665a97636ca8a2d0128b9da9d92c.

    Conflicts:
        include/gui/ISurfaceComposer.h
        libs/gui/ISurfaceComposer.cpp
        libs/gui/SurfaceComposerClient.cpp
        services/surfaceflinger/SurfaceFlinger.cpp
        services/surfaceflinger/SurfaceFlinger.h

Change-Id: Ia2be3a653049a01903c97fb73e44dd8d619fce6f
Support forcing all screenshots into a CPU consumer

Looks like a full GPU path is less efficient on some GPU
drivers that we're still using, and CPU is reliably faster...
(there's probably a locking condition going on somewhere, this
needs to be looked into)

Change-Id: I8878796a117d65bf2324507cf8755cadce49f6dc