[ot-font] Fallback vertical advance to UPEM

Used to be ascent+descent.  Whatever. This is more convenient for
upcoming changes.

src/hb-ot-font.cc

@@ -46,7 +46,7 @@ struct hb_ot_font_t
   {
     cmap.init (face);
     h_metrics.init (face);
-    v_metrics.init (face, h_metrics.ascender - h_metrics.descender); /* TODO Can we do this lazily? */
+    v_metrics.init (face);
 
     this->face = face;
     glyf.init ();

test/shaping/data/in-house/tests/vertical.tests

 ../fonts/191826b9643e3f124d865d617ae609db6a2ce203.ttf:--direction=t --font-funcs=ft:U+300C:[uni300C.vert=0@-512,-578+0,-1024]
 ../fonts/f9b1dd4dcb515e757789a22cb4241107746fd3d0.ttf:--direction=t --font-funcs=ft:U+0041,U+0042:[gid1=0@-654,-2128+0,-2789|gid2=1@-665,-2125+0,-2789]
-../fonts/f9b1dd4dcb515e757789a22cb4241107746fd3d0.ttf:--direction=t --font-funcs=ot:U+0041,U+0042:[gid1=0@-654,-2189+0,-2789|gid2=1@-665,-2189+0,-2789]
+../fonts/f9b1dd4dcb515e757789a22cb4241107746fd3d0.ttf:--direction=t --font-funcs=ot:U+0041,U+0042:[gid1=0@-654,-2189+0,-2048|gid2=1@-665,-2189+0,-2048]