Make LayerDim a regular Layer instead of a LayerBase

this is in preparation to get rid of LayerBase entirely

Change-Id: Ia051949fc5205fd87371331145356ee11598a597
diff --git a/services/surfaceflinger/Layer.h b/services/surfaceflinger/Layer.h
index 25afeef..e9eab17 100644
--- a/services/surfaceflinger/Layer.h
+++ b/services/surfaceflinger/Layer.h
@@ -68,7 +68,7 @@
     status_t setBuffers(uint32_t w, uint32_t h, 
             PixelFormat format, uint32_t flags=0);
 
-    bool isFixedSize() const;
+    virtual bool isFixedSize() const;
 
     // LayerBase interface
     virtual void setGeometry(const sp<const DisplayDevice>& hw,