Correctly handle translucency of device-specific pixel formats

Check requested format for device-specific formats, and assume (as
documented in libhardware/include/hardware/hardware.h) this is opaque
layer so no blending is necessary.

Bug: 3215931
Change-Id: Ib4dff8060ac522d201ff1e74807ac340c17d3fa7
diff --git a/services/surfaceflinger/Layer.h b/services/surfaceflinger/Layer.h
index d9a8be3..7538231 100644
--- a/services/surfaceflinger/Layer.h
+++ b/services/surfaceflinger/Layer.h
@@ -75,7 +75,8 @@
     virtual uint32_t doTransaction(uint32_t transactionFlags);
     virtual void lockPageFlip(bool& recomputeVisibleRegions);
     virtual void unlockPageFlip(const Transform& planeTransform, Region& outDirtyRegion);
-    virtual bool needsBlending() const      { return mNeedsBlending; }
+    virtual bool needsBlending(const sp<GraphicBuffer>& buffer) const;
+    virtual bool needsBlending() const;
     virtual bool needsDithering() const     { return mNeedsDithering; }
     virtual bool needsFiltering() const;
     virtual bool isSecure() const           { return mSecure; }