app_server: Scan fonts in the looper thread.

This way, the first unlucky application to request the font list
after paths change (or on first app_server startup) will not stall out
while the font list is rebuilt, at least most of the time.

Should fix #17574.

src/servers/app/font/FontManager.cpp

@@ -126,6 +126,9 @@ FontManager::FontManager()
 			// Precache the plain and bold fonts
 			_PrecacheFontFile(fDefaultPlainFont.Get());
 			_PrecacheFontFile(fDefaultBoldFont.Get());
+
+			// Post a message so we scan the initial paths.
+			PostMessage(B_PULSE);
 		}
 	}
 }
@@ -297,6 +300,9 @@ FontManager::MessageReceived(BMessage* message)
 			break;
 	}
 
+	// Scan fonts here if we need to, preventing other threads from having to do so.
+	_ScanFontsIfNecessary();
+
 	BLooper::MessageReceived(message);
 }