fix[2228133] pixelflinger ignores the "vertical stride" leading to artifacts when playing back video

we lost the concept of vertical stride when moving video playback to EGLImage.
Here we bring it back in a somewhat hacky-way that will work only for the
softgl/mdp backend.
diff --git a/include/ui/GraphicBuffer.h b/include/ui/GraphicBuffer.h
index 2172536..b9c491be 100644
--- a/include/ui/GraphicBuffer.h
+++ b/include/ui/GraphicBuffer.h
@@ -93,6 +93,8 @@
     
     void setIndex(int index);
     int getIndex() const;
+    void setVerticalStride(uint32_t vstride);
+    uint32_t getVerticalStride() const;
 
 protected:
     GraphicBuffer(const Parcel& reply);