Cache the Xft.dpi value outside

The Xft.dpi value is one way to set the dpi of the X11 display. Querying
this value requires much overhead. Therefore we have to cache this.

Previously we did just query it at the beginning and ignored further
changes.

As there is no native signal to catch a change in its xrdb value, we
have to rely on the root window's PropertyNotify event and filter there
the RESOURCE_MANAGER atom.

This will get hooked up later.

src/x11/screen.c

@@ -37,31 +37,42 @@ static int x_follow_tear_down_error_handler(void);
 static int FollowXErrorHandler(Display *display, XErrorEvent *e);
 static Window get_focused_window(void);
 
+
+/**
+ * A cache variable to cache the Xft.dpi xrdb values.
+ * We do not expect to change xrdb often, but there's much
+ * overhead to query it once.
+ *
+ * @retval -DBL_MAX: uncached
+ * @retval     <=0: Invalid and unusable value
+ * @retval      >0: valid
+ */
+double screen_dpi_xft_cache = -DBL_MAX;
+
+void screen_dpi_xft_cache_purge(void)
+{
+        screen_dpi_xft_cache = -DBL_MAX;
+}
+
 static double screen_dpi_get_from_xft(void)
 {
-        static double dpi = -1;
-        //Only run this once, we don't expect dpi changes during runtime
-        if (dpi <= -1) {
+        if (screen_dpi_xft_cache == -DBL_MAX) {
+                screen_dpi_xft_cache = 0;
+
                 XrmInitialize();
                 char *xRMS = XResourceManagerString(xctx.dpy);
 
-                if (!xRMS) {
-                        dpi = 0;
-                        return 0;
-                }
+                ASSERT_OR_RET(xRMS, screen_dpi_xft_cache);
 
                 XrmDatabase xDB = XrmGetStringDatabase(xRMS);
                 char *xrmType;
                 XrmValue xrmValue;
 
-                if (XrmGetResource(xDB, "Xft.dpi", "Xft.dpi", &xrmType, &xrmValue)) {
-                        dpi = strtod(xrmValue.addr, NULL);
-                } else {
-                        dpi = 0;
-                }
+                if (XrmGetResource(xDB, "Xft.dpi", "Xft.dpi", &xrmType, &xrmValue))
+                        screen_dpi_xft_cache = strtod(xrmValue.addr, NULL);
                 XrmDestroyDatabase(xDB);
         }
-        return dpi;
+        return screen_dpi_xft_cache;
 }
 
 static double screen_dpi_get_from_monitor(struct screen_info *scr)

src/x11/screen.h

@@ -17,6 +17,7 @@ struct screen_info {
 };
 
 void init_screens(void);
+void screen_dpi_xft_cache_purge(void);
 void screen_check_event(XEvent event);
 
 struct screen_info *get_active_screen(void);