Restore recently removed wxAuiGenericTabArt::m_tabCtrlHeight

This variable was removed as part of the changes of 1fa80f2 (Add
wxAUI_NB_MULTILINE for multiple tabs rows in wxAuiNotebook, 2025-01-06)
but it turns out that there is existing code relying on this member
variable existence, so preserve it for compatibility, even if this class
doesn't use it itself any longer.

Closes wxWidgets#25192.

Author: vadz

include/wx/aui/tabart.h

@@ -248,6 +248,7 @@ class WXDLLIMPEXP_AUI wxAuiGenericTabArt : public wxAuiTabArt
     wxBitmapBundle m_disabledWindowListBmp;
 
     int m_fixedTabWidth;
+    int m_tabCtrlHeight; // Unused, kept only for compatibility.
     unsigned int m_flags;
 
 private:

src/aui/tabart.cpp

@@ -193,6 +193,7 @@ wxAuiGenericTabArt::wxAuiGenericTabArt()
     m_measuringFont = m_selectedFont;
 
     m_fixedTabWidth = wxWindow::FromDIP(100, nullptr);
+    m_tabCtrlHeight = 0;
     m_flags = 0;
 
     UpdateColoursFromSystem();
@@ -278,6 +279,11 @@ void wxAuiGenericTabArt::SetSizingInfo(const wxSize& tab_ctrl_size,
         m_fixedTabWidth = tot_width/2;
 
     m_fixedTabWidth = wxMin(m_fixedTabWidth, wnd->FromDIP(220));
+
+    // We don't use this member variable ourselves any longer but keep it for
+    // compatibility with the existing code, deriving from this class and using
+    // it for its own purposes.
+    m_tabCtrlHeight = tab_ctrl_size.y;
 }