[open-type] In to_float() take offset as float

src/hb-open-type.hh

@@ -148,7 +148,7 @@ struct HBFixed : Type
   static_assert (Type::static_size * 8 > fraction_bits, "");
 
   HBFixed& operator = (typename Type::type i ) { Type::operator= (i); return *this; }
-  float to_float (int32_t offset = 0) const  { return ((int32_t) Type::v + offset) / shift; }
+  float to_float (float offset = 0) const  { return ((int32_t) Type::v + offset) / shift; }
   void set_float (float f) { Type::v = roundf (f * shift); }
   public:
   DEFINE_SIZE_STATIC (Type::static_size);