Keep buffer clipped to maximum in change_config.

Under a configuration change, where the bitrate suddenly decreases,
the buffer level may be larger than maximum allowed (for that first frame to be encoded after change_config).
This change keeps it clipped to its maximum level.

Change-Id: I4d0b5b3d1fd8148600dd39e02bd630c9464baba5

vp8/encoder/onyx_if.c

@@ -1623,6 +1623,12 @@ void vp8_change_config(VP8_COMP *cpi, VP8_CONFIG *oxcf)
         cpi->oxcf.maximum_buffer_size =
             rescale((int)cpi->oxcf.maximum_buffer_size,
                     cpi->oxcf.target_bandwidth, 1000);
+    // Under a configuration change, where maximum_buffer_size may change,
+    // keep buffer level clipped to the maximum allowed buffer size.
+    if (cpi->bits_off_target > cpi->oxcf.maximum_buffer_size) {
+      cpi->bits_off_target = cpi->oxcf.maximum_buffer_size;
+      cpi->buffer_level = cpi->bits_off_target;
+    }
 
     /* Set up frame rate and related parameters rate control values. */
     vp8_new_framerate(cpi, cpi->framerate);

vp9/encoder/vp9_onyx_if.c

@@ -1392,6 +1392,12 @@ void vp9_change_config(VP9_PTR ptr, VP9_CONFIG *oxcf) {
   else
     cpi->oxcf.maximum_buffer_size = rescale(cpi->oxcf.maximum_buffer_size,
                                             cpi->oxcf.target_bandwidth, 1000);
+  // Under a configuration change, where maximum_buffer_size may change,
+  // keep buffer level clipped to the maximum allowed buffer size.
+  if (cpi->rc.bits_off_target > cpi->oxcf.maximum_buffer_size) {
+    cpi->rc.bits_off_target = cpi->oxcf.maximum_buffer_size;
+    cpi->rc.buffer_level = cpi->rc.bits_off_target;
+  }
 
   // Set up frame rate and related parameters rate control values.
   vp9_new_framerate(cpi, cpi->oxcf.framerate);