Handling mask when converting wxBitmap to wxImage was accidentally
broken by 7e9afad53a
(Add real support for monochrome bitmaps to wxMSW,
2020-10-19). Fix this now and reuse the new wxMonoPixelData to make the
code simpler and more readable.
This commit is best viewed ignoring whitespace-only changes, as it
unindents a large block of code.
Closes https://github.com/wxWidgets/wxWidgets/pull/2125
Closes #18974, #18975.