surfaceflinger: add RenderEngine::setCurrentSurface

setCurrentSurface is a wrapper to eglMakeCurrent and it uses the EGL
context implied by the RenderEngine.  This also allows us to
simplify DisplayDevice::makeCurrent.

Test: SurfaceFlinger_test
Change-Id: Idab581f0ef79af7263159a558d8fad493a198ce7
diff --git a/services/surfaceflinger/RenderEngine/RenderEngine.h b/services/surfaceflinger/RenderEngine/RenderEngine.h
index b28454a..bd5eac2 100644
--- a/services/surfaceflinger/RenderEngine/RenderEngine.h
+++ b/services/surfaceflinger/RenderEngine/RenderEngine.h
@@ -99,6 +99,9 @@
         int getStatus() const;
     };
 
+    bool setCurrentSurface(EGLSurface surface);
+    void resetCurrentSurface();
+
     // set-up
     virtual void checkErrors() const;
     virtual void setViewportAndProjection(size_t vpw, size_t vph,