Fix border size computation in wxAuiTabArt.

Space was reserved for the borders even when it wasn't filled, resulting in
visual artefacts. Fix this by virtualizing the function returning the
additional space needed for the borders and only overriding it to return non
zero in wxAuiGtkTabArt.

Closes #14710.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72720 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/aui/tabart.h

@@ -103,6 +103,9 @@ public:
     virtual int GetBorderWidth(
                          wxWindow* wnd) = 0;
 
+    virtual int GetAdditionalBorderSpace(
+                         wxWindow* wnd) = 0;
+
     virtual int GetBestTabCtrlSize(
                          wxWindow* wnd,
                          const wxAuiNotebookPageArray& pages,
@@ -162,6 +165,9 @@ public:
     int GetBorderWidth(
                  wxWindow* wnd);
 
+    int GetAdditionalBorderSpace(
+                 wxWindow* wnd);
+
     wxSize GetTabSize(
                  wxDC& dc,
                  wxWindow* wnd,
@@ -258,6 +264,9 @@ public:
     int GetBorderWidth(
                  wxWindow* wnd);
 
+    int GetAdditionalBorderSpace(
+                 wxWindow* wnd);
+
     wxSize GetTabSize(
                  wxDC& dc,
                  wxWindow* wnd,