libgui: Bring back support for mHeap-based screenshots

Older graphics libraries throw a hissy fit when trying to lock
buffers for Surface-based screenshots, on at least Tegra2/3 and
Exynos4 hardware.

This patch depends on the BOARD_EGL_NEEDS_LEGACY_FB board flag
and requires the ro.bq.gpu_to_cpu_unsupported property set to 1
in order to work

Change-Id: I7db955e2cdd120018e349c14290e975788e70ed3
Revert "surfaceflinger: Don't take screenshots with legacy FB"

This reverts commit 245d3817b1979a44d53e67809b48d095ea6f43cc.
Merge branch 'cm-10.2' of git://github.com/CyanogenMod/android_frameworks_native into cm-10.2