video: Begin support for measuring multiple lines of text

Update the vidconsole API so that measure() can measure multiple lines
of text. This will make it easier to implement multi-line fields in
expo.

Tidy up the function comments while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/scene.c b/boot/scene.c
index 15e7a8b..d3ae581 100644
--- a/boot/scene.c
+++ b/boot/scene.c
@@ -298,7 +298,7 @@
 		}
 
 		ret = vidconsole_measure(scn->expo->cons, txt->font_name,
-					 txt->font_size, str, &bbox);
+					 txt->font_size, str, &bbox, NULL);
 		if (ret)
 			return log_msg_ret("mea", ret);
 		if (widthp)