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vp9-rtc: Fix to Q clamp in adjust_q_cbr

For CBR mode: clamp the Q to worst/best quality in
adjust_q_cbr().

Under certain conditions, when the worst/best quality is
suddenly changed by a large amount mid-stream, the Q
adjustment from the final Q from adjust_q_cbr may not respect
the worst/best qualiy limits.

Change-Id: I3776129325d89882d422b22e6247d44660dd90ac

--- a/vp9/encoder/vp9_ratectrl.c

+++ b/vp9/encoder/vp9_ratectrl.c

@@ -668,7 +668,7 @@

   }

   if (cpi->oxcf.content == VP9E_CONTENT_SCREEN)

     vp9_cyclic_refresh_limit_q(cpi, &q);

-  return q;

+  return VPXMAX(VPXMIN(q, cpi->rc.worst_quality), cpi->rc.best_quality);

 }

 static double get_rate_correction_factor(const VP9_COMP *cpi) {

@@ -1076,6 +1076,7 @@

         q = *top_index;

     }

   }

+

   assert(*top_index <= rc->worst_quality && *top_index >= rc->best_quality);

   assert(*bottom_index <= rc->worst_quality &&

          *bottom_index >= rc->best_quality);