vp8e - force at least some change in over and under shoots

Change-Id: Ie1796f272dc33bf5a1c8ac990da625961d272aa9

vp8/encoder/ratectrl.c

@@ -1532,6 +1532,15 @@ void vp8_compute_frame_size_bounds(VP8_COMP *cpi, int *frame_under_shoot_limit,
                 }
             }
         }
+
+        // For very small rate targets where the fractional adjustment
+        // (eg * 7/8) may be tiny make sure there is at least a minimum
+        // range.
+        *frame_over_shoot_limit += 200;
+        *frame_under_shoot_limit -= 200;
+        if ( *frame_under_shoot_limit < 0 )
+            *frame_under_shoot_limit = 0;
+
     }
 }