add:graphics:dpi scaling support (#813)

This commit adds calculating a scaling factor out of the virtual_dpi assumed when the layout was created, and the real_dpi value of the hardware. It is activated if "virtual_dpi" tag is set into the "graphics" tag on navit.xml.
Currently only qt5 graphics implment returning the hardware dpi. But this can be simulated and overwritten by setting "real_dpi" tag to "graphics" tag on navit.xm on all available graphics.l

navit/attr_def.h

@@ -200,6 +200,8 @@ ATTR(turn_around_penalty)
 ATTR(turn_around_penalty2)
 ATTR(autozoom_max)
 ATTR(nav_status)
+ATTR(virtual_dpi)
+ATTR(real_dpi)
 ATTR2(0x00027500,type_rel_abs_begin)
 /* These attributes are int that can either hold relative or absolute values. See the
  * documentation of ATTR_REL_RELSHIFT for details.

navit/callback.c

@@ -23,6 +23,7 @@
 #include "debug.h"
 #include "callback.h"
 
+
 struct callback {
     /* func has variable number of arguments, not (void),
      * but we must declare something... */
@@ -34,6 +35,8 @@ struct callback {
 };
 
 struct callback_list {
+    callback_patch patch;
+    void * patch_context;
     GList *list;
 };
 
@@ -115,6 +118,13 @@ void callback_list_remove_destroy(struct callback_list *l, struct callback *cb)
     g_free(cb);
 }
 
+void callback_list_add_patch_function (struct callback_list *l, callback_patch patch, void * context) {
+    if(!l)
+        return;
+    l->patch = patch;
+    l->patch_context = context;
+}
+
 void callback_call(struct callback *cb, int pcount, void **p) {
     int i;
     void *pf[8];
@@ -193,6 +203,8 @@ void callback_list_call_attr(struct callback_list *l, enum attr_type type, int p
     if (!l) {
         return;
     }
+    if(l->patch != NULL)
+        l->patch(l, type, pcount, p, l->patch_context);
 
     cbi=l->list;
     while (cbi) {

navit/callback.h

@@ -30,6 +30,7 @@ extern "C" {
 enum attr_type;
 struct callback;
 struct callback_list;
+typedef void (*callback_patch) (struct callback_list *l, enum attr_type type, int pcount, void **p, void * context);
 struct callback_list *callback_list_new(void);
 struct callback *callback_new_attr(void (*func)(void), enum attr_type type, int pcount, void **p);
 struct callback *callback_new_attr_args(void (*func)(void), enum attr_type type, int count, ...);
@@ -41,6 +42,7 @@ void callback_list_add(struct callback_list *l, struct callback *cb);
 struct callback *callback_list_add_new(struct callback_list *l, void (*func)(void), int pcount, void **p);
 void callback_list_remove(struct callback_list *l, struct callback *cb);
 void callback_list_remove_destroy(struct callback_list *l, struct callback *cb);
+void callback_list_add_patch_function (struct callback_list *l, callback_patch patch, void * context);
 void callback_call(struct callback *cb, int pcount, void **p);
 void callback_call_args(struct callback *cb, int count, ...);
 void callback_list_call_attr(struct callback_list *l, enum attr_type type, int pcount, void **p);