Problem:    Compiler warning on MS-Windows.
Solution:   Add type cast. (Mike Williams)

Problem:    Unnecessary changes in libvterm.
Solution:   Bring back // comments and trailing comma in enums.

Problem:    Unused macro argument in libvterm. (Randall W. Morris)
Solution:   Remove the argument.

Problem:    Libvterm can't handle a long OSC string that is split.
Solution:   When an incomplete OSC string is received copy it to the parser
            buffer. Increase the size of the parser buffer to be able to
            handle longer strings.

Problem:    Libvterm code lags behind master.
Solution:   Sync to head, solve merge problems.

Problem:    Compiler warning for visibility attribute not supported on MinGW
            builds.
Solution:   Don't add the attribute when we don't expect it to work.
            (Christian Brabandt)

Problem:    Cannot select text in a terminal with the mouse.
Solution:   When a job in a terminal is not consuming mouse events, use them
            for modeless selection.  Also stop Insert mode when clicking in a
            terminal window.

Problem:    Various small problems in source files.
Solution:   Fix the problems.

Problem:    Libterm doesn't support requesting fg and bg color.
Solution:   Implement t_RF and t_RB.

Problem:    Libvterm doesn't handle illegal byte sequence correctly.
Solution:   After the invalid code check if there is space to store another
            character.  Allocate one more character. (zhykzhykzhyk, closes
            #2614, closes #2613)