Add wxBitmapBundle::Clear()

This is just a convenient helper for resetting the bundle contents which
seems more readable than assigning an empty bundle to it.

include/wx/bmpbndl.h

@@ -108,6 +108,9 @@ public:
     // Check if bitmap bundle is non-empty.
     bool IsOk() const { return m_impl.get() != NULL; }
 
+    // Clear the bundle contents, IsOk() will return false after doing this.
+    void Clear() { m_impl.reset(NULL); }
+
     // Get the size of the bitmap represented by this bundle when using the
     // default DPI, i.e. 100% scaling. Returns invalid size for empty bundle.
     wxSize GetDefaultSize() const;

interface/wx/bmpbndl.h

@@ -288,6 +288,18 @@ public:
      */
     static wxBitmapBundle FromSVGResource(const wxString& name, const wxSize& sizeDef);
 
+    /**
+        Clear the existing bundle contents.
+
+        After calling this function IsOk() returns @false.
+
+        This is the same as assigning a default-constructed bitmap bundle to
+        this object but slightly more explicit.
+
+        @since 3.1.7
+     */
+    void Clear();
+
     /**
         Check if bitmap bundle is non-empty.