Force encoder to keep mbs in a lossless segment always lossless

Alt q segment feature is set as -255 for all lossless segments.
The purpose is to force the qindex of any block belonging to a
lossless segment not to be reverted to non-zero (lossy) when sb
level delta-q is also on.

BUG=aomedia:1925

Change-Id: I4ec399e323a8a9a4aba13637f314ed7256510046

av1/encoder/encodeframe.c

diff --git a/av1/encoder/encodeframe.c b/av1/encoder/encodeframe.c
index 80d7b5f..3f0b110 100644
--- a/av1/encoder/encodeframe.c
+++ b/av1/encoder/encodeframe.c
@@ -4291,6 +4291,10 @@
     xd->qindex[i] = qindex;
     if (xd->lossless[i]) {
       cpi->optimize_seg_arr[i] = 0;
+      // The purpose of setting alt q as -255 for all lossless segments is to
+      // force the qindex of any block belonging to a lossless segment not to be
+      // reverted to non-zero (lossy) when sb level delta-q is also on.
+      cm->seg.feature_data[i][SEG_LVL_ALT_Q] = -255;
     } else {
       cpi->optimize_seg_arr[i] = cpi->optimize_speed_feature;
     }