In order to be able to use monochrome bitmaps as wxMask, improve support for them in various ways: 1. Implement loading and saving of monochrome BMP files. 2. Add wxMonoPixelData for direct access to monochrome bitmap pixels. 3. Implement conversion from wxImage to monochrome wxBitmap. Closes https://github.com/wxWidgets/wxWidgets/pull/2032