Use correct size for the notebook background bitmap in wxMSW

Using "r.x + r.width" didn't really make much sense, it just happened to be
close enough to the real value to not create any problems when using LTR
layout, but was wrong in RTL, resulting in visible vertical line being visible
in the notebook client area.

Fix this using the same GetThemeBackgroundExtent() for determining the bitmap
size as is used for drawing over it.

src/msw/notebook.cpp

@@ -1151,9 +1151,26 @@ WXHBRUSH wxNotebook::QueryBgBitmap()
     if ( r.IsEmpty() )
         return 0;
 
+    wxUxThemeHandle theme(this, L"TAB");
+    if ( !theme )
+        return 0;
+
+    RECT rc;
+    wxCopyRectToRECT(r, rc);
+
     WindowHDC hDC(GetHwnd());
+    wxUxThemeEngine::Get()->GetThemeBackgroundExtent
+                            (
+                                theme,
+                                (HDC) hDC,
+                                9 /* TABP_PANE */,
+                                0,
+                                &rc,
+                                &rc
+                            );
+
     MemoryHDC hDCMem(hDC);
-    CompatibleBitmap hBmp(hDC, r.x + r.width, r.y + r.height);
+    CompatibleBitmap hBmp(hDC, rc.right, rc.bottom);
 
     SelectInHDC selectBmp(hDCMem, hBmp);