Account for the extra border around the thumb in wxMSW wxSlider

Without the extra margin, part of the thumb was truncated when
wxSL_TICKS was not used, but wxSL_BOTH (which makes the thumb bigger
than without it) was.

src/msw/slider.cpp

@@ -548,7 +548,11 @@ wxSize wxSlider::DoGetBestSize() const
 {
     // this value is arbitrary:
     const int length = FromDIP(100);
-    const int thumbSize = GetThumbLength();
+
+    // We need 2 extra pixels (which are not scaled by the DPI by the native
+    // control) on either side to account for the focus rectangle.
+    const int thumbSize = GetThumbLength() + 4;
+
     const int tickSize = FromDIP(TICK);
 
     int *width;