Add ext_newgrid and ext_hlstate extensions. These use predefined
highlights and line-segment based updates, for efficiency and
simplicity.. The ext_hlstate extension in addition allows semantic
identification of builtin and syntax highlights.

Reimplement the old char-based updates in the remote UI layer, for
compatibility. For the moment, this is still the default. The bulitin
TUI uses the new line-based protocol.

cmdline uses curwin cursor position when ext_cmdline is active.

Fix PVS warnings:

    fileio.c:7293   Medium  V547    Expression is always true.
    fileio.c:7351   Medium  V547    Expression 'event == 100' is always false.

event_name2nr returns event_T, so PVS infers that nothing outside of
that range could possibly be returned.

Preprocessor directives on the first line of the file were not being parsed.

also make termguicolors mutable after startup