rtc-screen: Adjust threshold for high moton detection

This reduces some dips in quality for scrolling with
scaled input.

Bug: b:444116924
Change-Id: I3cea6f59bdd4af439510d86c324c6344b53b2bb0

av1/encoder/ratectrl.c

diff --git a/av1/encoder/ratectrl.c b/av1/encoder/ratectrl.c
index ead6f34..7a2ea67 100644
--- a/av1/encoder/ratectrl.c
+++ b/av1/encoder/ratectrl.c
@@ -3348,8 +3348,12 @@
   }
   // Update the high_motion_content_screen_rtc flag on TL0. Avoid the update
   // if too many consecutive frame drops occurred.
+  // The threshold_high_motion is set to a large value, to account for
+  // mis-detection for scroll for scaled input (where scroll motion can be
+  // subpel and not detected below). The threshold may be reduced when better
+  // scroll detection (for subpel) is added.
   const int scale =
-      (unscaled_src->y_width * unscaled_src->y_height > 1920 * 1080) ? 24 : 10;
+      (unscaled_src->y_width * unscaled_src->y_height >= 1920 * 1080) ? 40 : 16;
   const uint64_t thresh_high_motion = scale * 64 * 64;
   if (cpi->svc.temporal_layer_id == 0 && rc->drop_count_consec < 3) {
     cpi->rc.high_motion_content_screen_rtc = 0;