multi window shadow for win32 and macos

[Antoine Martin]

Same as #1801 was for X11 shadow servers.

For win32, the are some pointers in #389.

See also #1825 for multi-window support done on Linux.

[Antoine Martin]

Done fairly generically in r19268. Tested OK on win32 with dual monitors, including monitors left-of-primary which ends up using negative coordinates.

This should also work on macos, assuming that GTK sees what macos calls "screens". (otherwise we already have native code for that)
I can't test on macos because virtualbox's multiple monitors aren't detected, and the macmini doesn't have the monitor or cables required.

[Antoine Martin]

Done for macos in r19600.

Still TODO:

• should the win32 shadow servers also re-use the same adjust_pointer code as macos and x11? (then refactor it up)

[Antoine Martin]

• adjust pointer code did need refactoring, done in r19602
• also fix adjusting pointer location reported back to the client: r19603