Allow block skip coding option in RTC mode

When the estimated rate-distortion cost of skip coding mode is
lower than that of sending quantized coefficients, allow the
encoder to drop these coefficients. This improves the compression
performance of speed -6 by 0.268% and makes the encoding speed
slightly faster.

Change-Id: Idff2d7ba59f27ead33dd5a0e9f68746ed3c2ab68

vp9/encoder/vp9_pickmode.c

@@ -1082,6 +1082,7 @@ void vp9_pick_inter_mode(VP9_COMP *cpi, MACROBLOCK *x,
         } else {
           this_rdc.rate = vp9_cost_bit(vp9_get_skip_prob(cm, xd), 1);
           this_rdc.dist = this_sse;
+          x->skip_txfm[0] = 1;
         }
       }