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/libs/ui/GraphicBuffer.cpp b/libs/ui/GraphicBuffer.cpp
index efe2d78..6a5c8a9 100644
--- a/libs/ui/GraphicBuffer.cpp
+++ b/libs/ui/GraphicBuffer.cpp
@@ -226,6 +226,14 @@
     return mIndex;
 }
 
+void GraphicBuffer::setVerticalStride(uint32_t vstride) {
+    mVStride = vstride;
+}
+
+uint32_t GraphicBuffer::getVerticalStride() const {
+    return mVStride;
+}
+
 // ---------------------------------------------------------------------------
 
 }; // namespace android