Document buffers in MACROBLOCK

This CL adds a brief description for the buffers in MACROBLOCK. In
addition, the four buffers used to speed up obmc search are refactored to
a new struct called OBMCBuffer.

BUG=aoemdia:2618

Change-Id: Ibb9e57c359504c81af9c6aa50ccf880901f7c5c3
diff --git a/av1/encoder/mcomp.c b/av1/encoder/mcomp.c
index ffe014c..8827486 100644
--- a/av1/encoder/mcomp.c
+++ b/av1/encoder/mcomp.c
@@ -51,8 +51,8 @@
 
   av1_set_ms_compound_refs(ms_buffers, NULL, NULL, 0, 0);
 
-  ms_buffers->wsrc = x->wsrc_buf;
-  ms_buffers->obmc_mask = x->mask_buf;
+  ms_buffers->wsrc = x->obmc_buffer.wsrc;
+  ms_buffers->obmc_mask = x->obmc_buffer.mask;
 }
 
 void av1_make_default_fullpel_ms_params(FULLPEL_MOTION_SEARCH_PARAMS *ms_params,