Clean out more MSVC warnings

Change-Id: I1bab0c104df2ec4825d050cd516e26ab635a7b3e
diff --git a/tools_common.c b/tools_common.c
index 901734e..8d356af 100644
--- a/tools_common.c
+++ b/tools_common.c
@@ -392,7 +392,7 @@
           (uint16_t *)(src->planes[plane] + y * src->stride[plane]);
       uint8_t *p_dst = dst->planes[plane] + y * dst->stride[plane];
       for (x = 0; x < w; x++) {
-        *p_dst++ = *p_src++;
+        *p_dst++ = (uint8_t)(*p_src++);
       }
     }
   }