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.
Make HlAttr contain highlighting state for both color modes (cterm and rgb).
This allows us to implement termguicolors completely in the TUI.
Simplify some logic duplicated between ui.c and screen.c. Also avoid
some superfluous highlighting reset events.
