Handle negative width size in GDI+ DrawRectangle() and document it

Apparently it's a common convention to allow width and/or height of a
rectangle to be negative as both GDI and Cairo handle this natively, so also
allow this for GDI+ and document this as the expected behaviour.

Closes #17495.

interface/wx/dc.h

@@ -523,8 +523,14 @@ public:
                          wxPolygonFillMode fill_style = wxODDEVEN_RULE);
 
     /**
-        Draws a rectangle with the given top left corner, and with the given
+        Draws a rectangle with the given corner coordinate and size.
-        size.  The current pen is used for the outline and the current brush
+
+        Normally, @a x and @a y specify the top left corner coordinates and
+        both @a width and @a height are positive, however they are also allowed
+        to be negative, in which case the corresponding corner coordinate
+        refers to the right or bottom corner instead.
+
+        The current pen is used for the outline and the current brush
         for filling the shape.
     */
     void DrawRectangle(wxCoord x, wxCoord y, wxCoord width, wxCoord height);

src/msw/graphics.cpp

@@ -1508,6 +1508,18 @@ void wxGDIPlusContext::DrawRectangle( wxDouble x, wxDouble y, wxDouble w, wxDoub
     Brush *brush = m_brush.IsNull() ? NULL : ((wxGDIPlusBrushData*)m_brush.GetRefData())->GetGDIPlusBrush();
     Pen *pen = m_pen.IsNull() ? NULL : ((wxGDIPlusPenData*)m_pen.GetGraphicsData())->GetGDIPlusPen();
 
+    if ( w < 0 )
+    {
+        x += w;
+        w = -w;
+    }
+
+    if ( h < 0 )
+    {
+        y += h;
+        h = -h;
+    }
+
     if ( brush )
     {
         // the offset is used to fill only the inside of the rectangle and not paint underneath