xcb: implement QXcbCursor::size()

A follow up of baed8534bc1dac36a9d0ef4240fc14398076a192 .

Fixes: QTBUG-118612
Pick-to: 6.5 6.2
Change-Id: I7b4a2276c6e6a14d5413f6cd64d801de11cc8c3e
Reviewed-by: JiDe Zhang <zhangjide@uniontech.com>
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit 78272c667f008f6afcf5f84e0add525b56c3a558)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/xcb/qxcbcursor.cpp

@@ -10,6 +10,7 @@
 #include <QtGui/QWindow>
 #include <QtGui/QBitmap>
 #include <QtGui/private/qguiapplication_p.h>
+#include <qpa/qplatformtheme.h>
 
 #if QT_CONFIG(xcb_xlib)
 #include <X11/cursorfont.h>
@@ -288,6 +289,13 @@ QXcbCursor::~QXcbCursor()
         xcb_cursor_context_free(m_cursorContext);
 }
 
+QSize QXcbCursor::size() const
+{
+    if (const QPlatformTheme *theme = QGuiApplicationPrivate::platformTheme())
+        return theme->themeHint(QPlatformTheme::MouseCursorSize).toSize();
+    return QSize(24, 24);
+}
+
 void QXcbCursor::updateContext()
 {
     if (m_cursorContext)

src/plugins/platforms/xcb/qxcbcursor.h

@@ -48,6 +48,8 @@ public:
     QPoint pos() const override;
     void setPos(const QPoint &pos) override;
 
+    QSize size() const override;
+
     void updateContext();
 
     static void queryPointer(QXcbConnection *c, QXcbVirtualDesktop **virtualDesktop, QPoint *pos, int *keybMask = nullptr);