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

Change-Id: Icd12ae388077b8c9ed329c37314e896500078543
(cherry picked from commit 48b6a9767317e08a24e189c9cbee21043543682a)
(cherry picked from commit 777610a5ddf19cebcad30b7e9a481e9dfe38b74c)
Update the layer's alpha value upon composition of the layer

Fixes: This patch makes sure that the layer's alpha value is
up-to-date and does not reflect the previous view's alpha value.
It fixes the square block on marquee fading edge when text view
is applied transparency.

CRs-Fixed: 562000

Change-Id: I397308b739f5f4fe4b630cf594825f401c97e0e3
Fix stencil buffer bug.

Stencil test is enabled by setStencilFromClip(), but not disabled
in some use case, which causes some of the pixels are discarded by
mistake in draws followed. Disable stencil test when there is no
dirty clip.

CRs-Fixed: 539456

Change-Id: I318fb47212e49e2da89bcbfd161a1689ef91b02f