Add fallback cursor shape for "default" shape

Some cursor themes don't have "default" shape, so fallback to "left_ptr"
with such themes.

BUG: 477476

src/cursor.cpp

@@ -349,6 +349,12 @@ QList<QByteArray> Cursor::cursorAlternativeNames(const QByteArray &name)
                 QByteArrayLiteral("cross-reverse"),
             },
         },
+        {
+            QByteArrayLiteral("default"),
+            {
+                QByteArrayLiteral("left_ptr"),
+            },
+        },
         {
             QByteArrayLiteral("up_arrow"),
             {