Add wxSize::DecToIfSpecified() helper.

This is similar to DecTo() but can be used even if the wxSize argument has
some components set to -1.

Notice that we don't need IncToIfSpecified() because IncTo() already works
correctly anyhow in this case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72342 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/gdicmn.h

@@ -873,6 +873,18 @@ public:
     */
     void DecTo(const wxSize& size);
 
+    /**
+        Decrements this object to be not bigger than the given size ignoring
+        non-specified components.
+
+        This is similar to DecTo() but doesn't do anything for x or y
+        component if the same component of @a size is not specified, i.e. set
+        to ::wxDefaultCoord.
+
+        @since 2.9.5
+     */
+    void DecToIfSpecified(const wxSize& size);
+
     /**
         Gets the height member.
     */