Don't reset mb clamping state during splitmv decoding

The MV decoding changes in c5fb0eb8 introduced a bug where the
macroblock clamping state was reset for each partition, so if an
earlier partition needed clamping but a subsequent one didn't,
the MB wouldn't receive clamping. Instead, the state is only
set during splitmv decoding, never cleared.

Change-Id: I224fe258493405ee0f6a04596acdb622c475e845

vp8/decoder/decodemv.c

@@ -394,7 +394,7 @@ void vp8_read_mb_modes_mv(VP8D_COMP *pbi, MODE_INFO *mi, MB_MODE_INFO *mbmi,
                     break;
                 }
 
-                mbmi->need_to_clamp_mvs = (mv->col < mb_to_left_edge) ? 1 : 0;
+                mbmi->need_to_clamp_mvs |= (mv->col < mb_to_left_edge) ? 1 : 0;
                 mbmi->need_to_clamp_mvs |= (mv->col > mb_to_right_edge) ? 1 : 0;
                 mbmi->need_to_clamp_mvs |= (mv->row < mb_to_top_edge) ? 1 : 0;
                 mbmi->need_to_clamp_mvs |= (mv->row > mb_to_bottom_edge) ? 1 : 0;