Remove opaque check in preparing dirty region

Since preserve swap is enabled, we need to clear the color buffer
when the scissor rect is prepared for a new process. This prevents
garbage being present from the previous process in the color buffer.

CRs-Fixed: 549755

Conflicts:
	libs/hwui/OpenGLRenderer.cpp

Change-Id: Icd12ae388077b8c9ed329c37314e896500078543
(cherry picked from commit 48b6a9767317e08a24e189c9cbee21043543682a)
(cherry picked from commit 777610a5ddf19cebcad30b7e9a481e9dfe38b74c)