refactored screenshot code

the core screenshot function now can capture the screen at any lower resolution
performing bilinear filtering.

we also now have some client code to interface with the screenshot service.

it's now possible to request a screenshot at a lower resolution.

Change-Id: I33689bba98507ab928d0898b21596d0d2fe4b953
diff --git a/services/surfaceflinger/Layer.h b/services/surfaceflinger/Layer.h
index e1d283b..263c372 100644
--- a/services/surfaceflinger/Layer.h
+++ b/services/surfaceflinger/Layer.h
@@ -68,6 +68,7 @@
     bool isFixedSize() const;
 
     // LayerBase interface
+    virtual void drawForSreenShot() const;
     virtual void onDraw(const Region& clip) const;
     virtual uint32_t doTransaction(uint32_t transactionFlags);
     virtual void lockPageFlip(bool& recomputeVisibleRegions);