unmultiply by alpha when converting from wxDIB to wxImage as the former uses premultiplied alpha but the latter does not (patch 1597063)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@43511 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -98,6 +98,7 @@ wxMSW:
 
 - Fixed version script problems when using configure with cygwin/mingw32.
 - Use system default paper size for printing instead of A4
+- Fix colours when converting wxBitmap with alpha to wxImage (nusi)
 
 2.7.2
 -----

src/msw/dib.cpp

@@ -775,14 +775,26 @@ wxImage wxDIB::ConvertToImage() const
             dst[1] = *src++;
             dst[0] = *src++;
 
-            dst += 3;
-
             if ( is32bit )
             {
                 if ( alpha )
-                    *alpha++ = *src;
+                {
+                    // wxImage uses non premultiplied alpha so undo
+                    // premultiplication done in Create() above
+                    const unsigned char a = *src;
+                    *alpha++ = a;
+                    if ( a > 0 )
+                    {
+                        dst[0] = (dst[0] * 255 - 127) / a;
+                        dst[1] = (dst[1] * 255 - 127) / a;
+                        dst[2] = (dst[2] * 255 - 127) / a;
+                    }
+                }
+
                 src++;
             }
+
+            dst += 3;
         }
 
         // pass to the previous line in the image