Fix creating a bitmap representing wxMask

When bitmap data are accessed with wxNativePixelData, which is designed
to handle RGB bitmaps, Alpha() function cannot be used because in this
case alpha component index is set to the default value -1 and actually
the blue component of the "preceding" pixel is accessed.

Closes #18478.

src/osx/core/bitmap.cpp

@@ -1697,10 +1697,8 @@ wxBitmap wxMask::GetBitmap() const
         for (int x = 0; x < m_width; ++x, ++p, ++src)
         {
             const unsigned char byte = *src;
-            p.Alpha() = 0xff;
-            p.Red() = byte;
-            p.Green() = byte;
-            p.Blue() = byte;
+            wxASSERT( byte == 0 || byte == 0xFF );
+            p.Red() = p.Green() = p.Blue() = byte;
         }
         p = rowStart;
         p.OffsetY(data, 1);