libge2d: add support for independent alpha channel [1/1]

PD#SWPL-151902

Problem:
need support for 'nv12 + alpha -> rgba'

Solution:
complete this support

Verify:
sc2

Change-Id: I8e33e0f24b82eca8a59c7b3cf1339d1565fef022
Signed-off-by: Jian Cao <jian.cao@amlogic.com>
3 files changed