src/definition/structuredrichtext: use primary screen when current null

When the current screen is null, use the primary screen from
QGuiApplication to get the DPI. This fixes the first result being
rendered with the wrong DPI in most cases. It is kind of hard to fix in
cases where screens with multiple DPIs are mixed and matched, but those
are niche. This works in the general case.

This commit was written by Codex and edited by me.

Author: ripose-jp

src/definition/structuredrichtext.cpp

@@ -23,6 +23,7 @@
 #include <algorithm>
 #include <cmath>
 
+#include <QGuiApplication>
 #include <QJsonObject>
 #include <QQuickWindow>
 #include <QSet>
@@ -2071,7 +2072,11 @@ double StructuredRichText::screenDpi(const QScreen *screen) const
 
     if (screen == nullptr)
     {
-        return DEFAULT_SCREEN_DPI;
+        screen = QGuiApplication::primaryScreen();
+        if (screen == nullptr)
+        {
+            return DEFAULT_SCREEN_DPI;
+        }
     }
     const double dpi = screen->logicalDotsPerInch();
     return dpi > 0.0 ? dpi : DEFAULT_SCREEN_DPI;