Changed wxImage::ConvertAlphaToMask() return type to bool.

Make this function more useful by returning true from it if alpha channel was
really converted to the mask by it.

Closes #12637.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65990 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/image.h

@@ -815,7 +815,7 @@ public:
         FindFirstUnusedColour() by this function, see the overload below if you
         this is not appropriate.
 
-        @return @false if FindFirstUnusedColour returns @false, @true otherwise.
+        @return Returns @true on success, @false on error.
     */
     bool ConvertAlphaToMask(unsigned char threshold = wxIMAGE_ALPHA_THRESHOLD);
 
@@ -841,8 +841,9 @@ public:
             are set. Pixels with the alpha values above the threshold are
             considered to be opaque.
 
+        @return Returns @true on success, @false on error.
      */
-    void ConvertAlphaToMask(unsigned char mr, unsigned char mg, unsigned char mb,
+    bool ConvertAlphaToMask(unsigned char mr, unsigned char mg, unsigned char mb,
                             unsigned char threshold = wxIMAGE_ALPHA_THRESHOLD);
 
     /**