fix a bug that "optimize" flag is not set for sub-threads

The flag for quantization optimization was not properly propagated to
mb row encoding threads.

Change-Id: Ic561599c35acd94cd5698c9b314bccd596ac2deb

vp8/encoder/ethreading.c

@@ -281,6 +281,7 @@ static void setup_mbby_copy(MACROBLOCK *mbdst, MACROBLOCK *mbsrc)
     z->vp8_short_fdct8x4     = x->vp8_short_fdct8x4;
     z->short_walsh4x4    = x->short_walsh4x4;
     z->quantize_b        = x->quantize_b;
+    z->optimize          = x->optimize;
 
     /*
     z->mvc              = x->mvc;