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vp9: limit lpf workers to min(threads,tiles,sb_rows)

this implementation does not scale well beyond that. this restores the
performance in v1.7.0.

BUG=webm:1574

Change-Id: I8f3464cfe871988fa06ebefe9954811fd002584e

--- a/vp9/common/vp9_thread_common.c

+++ b/vp9/common/vp9_thread_common.c

@@ -159,7 +159,12 @@

   const VPxWorkerInterface *const winterface = vpx_get_worker_interface();

   // Number of superblock rows and cols

   const int sb_rows = mi_cols_aligned_to_sb(cm->mi_rows) >> MI_BLOCK_SIZE_LOG2;

-  const int num_workers = VPXMIN(nworkers, sb_rows);

+  const int num_tile_cols = 1 << cm->log2_tile_cols;

+  // Limit the number of workers to prevent changes in frame dimensions from

+  // causing incorrect sync calculations when sb_rows < threads/tile_cols.

+  // Further restrict them by the number of tile columns should the user

+  // request more as this implementation doesn't scale well beyond that.

+  const int num_workers = VPXMIN(nworkers, VPXMIN(num_tile_cols, sb_rows));

   int i;

   if (!lf_sync->sync_range || sb_rows != lf_sync->rows ||