Add bool return value for wxDC::GetClippingBox()

Determining whether there is an actual clipping region or not is not
that simple, as shown by the recent problems in wxDCClipper code, so
return a boolean value indicating this from GetClippingBox() directly,
instead of requiring the caller to find it out on their own.

This simplifies wxDCClipper code, as well as any other code calling
GetClippingBox(), at the price of some extra complexity in wxDCImpl
itself, which seems to be worth it.

interface/wx/dc.h

@@ -764,8 +764,30 @@ public:
 
         @remarks
         Clipping region is given in logical coordinates.
+
+        @param x If non-@NULL, filled in with the logical horizontal coordinate
+            of the top left corner of the clipping region if the function
+            returns true or 0 otherwise.
+        @param y If non-@NULL, filled in with the logical vertical coordinate
+            of the top left corner of the clipping region if the function
+            returns true or 0 otherwise.
+        @param width If non-@NULL, filled in with the width of the clipping
+            region if the function returns true or the device context width
+            otherwise.
+        @param height If non-@NULL, filled in with the height of the clipping
+            region if the function returns true or the device context height
+            otherwise.
+        @return @true if there is a clipping region or @false if there is no
+            active clipping region (note that this return value is available
+            only since wxWidgets 3.1.2, this function didn't return anything in
+            the previous versions).
     */
-    void GetClippingBox(wxCoord *x, wxCoord *y, wxCoord *width, wxCoord *height) const;
+    bool GetClippingBox(wxCoord *x, wxCoord *y, wxCoord *width, wxCoord *height) const;
+
+    /**
+        @overload
+    */
+    bool GetClippingBox(wxRect& rect) const;
 
     /**
         Sets the clipping region for this device context to the intersection of