Added per pixel inter rd hit count stats

Added some code to output normalized rd hit count stats.
In effect this approximates to the average number of rd
operations/tests per pixel for the sequence.

The results are not quite accurate and I have not bothered
to account for partial SB64s at frame edges and for key frames
However they do give some idea of the number of modes /
prediction methods being tested for each pixel across the
different partition sizes. This indicates how much scope their
is for further gains either by reducing the number of partitions
examined or the modes per partition through heuristics.

Patch 3 moved place where count incremented so partial rd
tests that are aborted with INT_MAX return are also counted.

Example numbers for first 50 frames of Akiyo.
Speed 0 ~84.4 rd operations / pixel
Speed 1 ~28.8
Speed 2 ~11.9

Change-Id: Ib956e787e12f7fa8b12d3a1a2f6cda19a65a6cb8

vp9/encoder/vp9_rdopt.c

@@ -3341,6 +3341,12 @@ int64_t vp9_rd_pick_inter_mode_sb(VP9_COMP *cpi, MACROBLOCK *x,
       continue;
     }
 
+#ifdef MODE_TEST_HIT_STATS
+    // TEST/DEBUG CODE
+    // Keep a rcord of the number of test hits at each size
+    cpi->mode_test_hits[bsize]++;
+#endif
+
     if (this_mode == I4X4_PRED) {
       int rate;