Refactor segmentation related params in AV1_COMP

This CL groups together segmentation related params in AV1_COMP into
a new struct EncSegmentationInfo, and adds documentation for the
members.

BUG=aomedia:2610

Change-Id: Ib14e451b1f50b5ae7f9f7ef1fa6ec61c66afc8a1
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index aafa9fd..daa8ce1 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -455,11 +455,11 @@
     // Still need to transmit tx size for intra blocks even if skip is
     // true. Changing segment_id may make the tx size become invalid, e.g
     // changing from lossless to lossy.
-    assert(is_inter_block(mbmi) || !cpi->has_lossless_segment);
+    assert(is_inter_block(mbmi) || !cpi->enc_seg.has_lossless_segment);
 
     set_spatial_segment_id(&cm->mi_params, cm->cur_frame->seg_map,
                            mbmi->sb_type, mi_row, mi_col, pred);
-    set_spatial_segment_id(&cm->mi_params, cpi->segmentation_map, mbmi->sb_type,
+    set_spatial_segment_id(&cm->mi_params, cpi->enc_seg.map, mbmi->sb_type,
                            mi_row, mi_col, pred);
     /* mbmi is read only but we need to update segment_id */
     ((MB_MODE_INFO *)mbmi)->segment_id = pred;