Performance improvement for 2D for large screen devices

On large screen devices copying the full front buffer to backbuffer
cause performance degradation. Hence we propose to copy only the dirty
region. Given this is memory intensive operation we suggest to do it only
when the dirty region is smaller than half of the screen size. We
have observed avg 40% performance improvement in 2D rendering with
this patch on our devices.

Signed-off-by: Raj Mamadgi <rmamadgi@sta.samsung.com>

Change-Id: I8f689685a38daf2e221c312234c3fbf93021fcfc