Allow rescaling larger images in Win64 builds

Use wxUIntPtr rather than (unsigned) long in wxImage::ResampleNearest()
as long is still 32 bits under Win64 and so doesn't allow the code there
to work with images larger than 2^16 in either direction, when it could
be allowed in this case.

Document the current limits on the size of the image and add a unit test
checking that resizing images of size greater than 2^16 works in 64 bits.

See #18550.

interface/wx/image.h

@@ -972,6 +972,11 @@ public:
         }
         @endcode
 
+        @note The algorithm used for the default (normal) quality value doesn't
+        work with images larger than 65536 (2^16) pixels in either dimension
+        for 32-bit programs. For 64-bit programs the limit is 2^48 and so not
+        relevant in practice.
+
         @see Rescale()
     */
     wxImage Scale(int width, int height,