Refactor bgsprite.

Changes:
- Refactored alignment and blending out of stitch_image function
- Variable name changes in stitch_image to increase readability
- Changes to temporal_filter (under CONFIG_BGSPRITE flag) in preparation
for blending of bgsprite and temporal filtered arf.

Change-Id: I3c4e76f26d6586684079ae7e666881b6ce14b2db
diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index 24c6caa..1faa0ec 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -5739,7 +5739,7 @@
 #endif  // CONFIG_BGSPRITE
           av1_temporal_filter(cpi,
 #if CONFIG_BGSPRITE
-                              NULL,
+                              NULL, &cpi->alt_ref_buffer,
 #endif  // CONFIG_BGSPRITE
                               arf_src_index);
         aom_extend_frame_borders(&cpi->alt_ref_buffer);