Round, instead of floor, when applying variations

Hoping this would fix remaining text-rendering-tests failures,
but so far no luck.

src/hb-font-private.hh

@@ -541,7 +541,7 @@ struct hb_font_t {
   }
   inline hb_position_t em_scalef (float v, int scale)
   {
-    return (hb_position_t) (v * scale / face->get_upem ());
+    return (hb_position_t) round (v * scale / face->get_upem ());
   }
 };
 

src/hb-private.hh

@@ -40,6 +40,7 @@
 #define HB_OT_H_IN
 #endif
 
+#include <math.h>
 #include <stdlib.h>
 #include <stddef.h>
 #include <string.h>