st: Add high dpi support

Add a scale_factor property to StThemeContext that can
be used to enable (integer) scaling of pixel values.

https://bugzilla.gnome.org/show_bug.cgi?id=705410

src/st/st-theme-node.c

@@ -1015,6 +1015,9 @@ get_length_from_term (StThemeNode *node,
   } type = ABSOLUTE;
 
   double multiplier = 1.0;
+  int scale_factor;
+
+  g_object_get (node->context, "scale-factor", &scale_factor, NULL);
 
   if (term->type != TERM_NUMBER)
     {
@@ -1028,7 +1031,7 @@ get_length_from_term (StThemeNode *node,
     {
     case NUM_LENGTH_PX:
       type = ABSOLUTE;
-      multiplier = 1;
+      multiplier = 1 * scale_factor;
       break;
     case NUM_LENGTH_PT:
       type = POINTS;