separate mHeap-based screenshot from BOARD_EGL_NEEDS_LEGACY_FB

No need for the Exynos/Tegra legacy fb implementation on legacy
qcom. Add a separate flag for mHeap-based screenshot in case of
future extensions. Enable mHeap-based screenshot for legacy qcom
dislay.

Change-Id: I35ff015555f19a5ba2857ad3d765d02eb6b9acde

Conflicts:

	libs/gui/Android.mk
Forward-port surface dithering from CM10

This allows the use of 16-bit color displays without excessive color
banding; behavior can be controlled by the persist.sys.use_dithering
property.

Change-Id: I290ca4ed4787dc81ac756d9af92cd6a690865a08

Conflicts:

	services/surfaceflinger/SurfaceFlinger.cpp
	services/surfaceflinger/SurfaceFlinger.h