make rdmult adaptive for intra in quantizer RDO

This intends to correct the tendency that VP8 aggressively favors rate
on intra coded frames. Experiments tested different numbers in [0, 1]
and found 9/16 overall provided about 2-4% gains for all-intra coded
clips based on vpx-ssim metric. The impact on regular encoded clips
is much smaller but positive overall. Overall impact on psnr is also
positive even though very small.

Change-Id: If808553aaaa87fdd44691f9787820ac9856d9f8a

vp8/encoder/encodemb.c

@@ -309,8 +309,10 @@ void vp8_optimize_b(MACROBLOCK *mb, int ib, int type,
     eob = d->eob;
 
     /* Now set up a Viterbi trellis to evaluate alternative roundings. */
-    /* TODO: These should vary with the block type, since the quantizer does. */
     rdmult = (mb->rdmult << 2)*err_mult;
+    if(mb->e_mbd.mode_info_context->mbmi.ref_frame==INTRA_FRAME)
+        rdmult = (rdmult * 9)>>4;
+
     rddiv = mb->rddiv;
     best_mask[0] = best_mask[1] = 0;
     /* Initialize the sentinel node of the trellis. */