fix custom mouse cursors

UI elements with custom mouse cursors wouldn't show them due to the cursor
being reset at the start of each frame. To fix this we need to be sure that
we only reset when the element is no longer active.

Author: notimaginative

code/ui/button.cpp

@@ -445,7 +445,7 @@ void UI_BUTTON::maybe_show_custom_cursor()
 
 void UI_BUTTON::restore_previous_cursor()
 {
-	if (previous_cursor != NULL) {
+	if (previous_cursor != NULL && !is_mouse_on()) {
 		io::mouse::CursorManager::get()->setCurrentCursor(previous_cursor);
 		previous_cursor = NULL;
 	}