0911/s6 orient unblunted on pr5275

[chengchen-google]

Support Level 2 orientation partition pruning for 4K at Speed >= 6

The performance on RA, speed 6, A1:

BD-Rate (YUV)  Speedup      Ratio
+1.09%         +14.32%       13.1

STATS_CHANGED

[chengchen-google]

Note that this PR is a follow-up of #5275

The diff is much smaller than the patch shows.
Diff:

-  const uint64_t frame_scaled = orientation_anisotropy_for_frame(cpi, 1);
-  if (frame_scaled == 0) return;
-  const uint64_t anisotropy =
-      orientation_anisotropy_for_bsize(bsize, base_anisotropy) * frame_scaled;
+  uint64_t anisotropy;
+  if (cpi->sf.part_sf.prune_by_struct_orient >= 2) {
+    // Level 2 (unblunted mode for 4K at speed >= 6): prune across all frames
+    // regardless of temporal layer depth (patch 0006d).
+    anisotropy = base_anisotropy;
+  } else {
+    const uint64_t frame_scaled = orientation_anisotropy_for_frame(cpi, 1);
+    if (frame_scaled == 0) return;
+    anisotropy =
+        orientation_anisotropy_for_bsize(bsize, base_anisotropy) * frame_scaled;
+  }

+    if (is_4k_or_larger) {
+      sf->part_sf.prune_by_struct_orient = 2;
+    }

-  bool prune_by_struct_orient;
+  // Prune rectangular and extended partitions based on structure orientation
+  // of the source signal.
+  // 0: disabled
+  // 1: standard (size-graded anisotropy with GOP temporal layer protection)
+  // 2: unblunted (prunes across all GOP temporal layers for 4K at speed >= 6)
+  int prune_by_struct_orient;

[urvangjoshi]

@chengchen-google : I believe this PR depends on the base PR? If so, @yeqing-wu could you review the base PR trade-off?

Also @chengchen-google : the PR title / description will become commit message after merging, so pls edit it.