ui: use line-based rather than char-based updates in screen.c

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.

src/nvim/ui.h

@@ -5,13 +5,18 @@
 #include <stdbool.h>
 #include <stdint.h>
 
-#include "api/private/defs.h"
+#include "nvim/globals.h"
+#include "nvim/api/private/defs.h"
+#include "nvim/highlight_defs.h"
 
 typedef enum {
   kUICmdline = 0,
   kUIPopupmenu,
   kUITabline,
   kUIWildmenu,
+#define kUIGlobalCount (kUIWildmenu+1)
+  kUINewgrid,
+  kUIHlState,
   kUIExtCount,
 } UIExtension;
 
@@ -19,7 +24,9 @@ EXTERN const char *ui_ext_names[] INIT(= {
   "ext_cmdline",
   "ext_popupmenu",
   "ext_tabline",
-  "ext_wildmenu"
+  "ext_wildmenu",
+  "ext_newgrid",
+  "ext_hlstate",
 });
 
 
@@ -30,9 +37,17 @@ struct ui_t {
   bool ui_ext[kUIExtCount];  ///< Externalized widgets
   int width, height;
   void *data;
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "ui_events.generated.h"
 #endif
+
+  // For perfomance and simplicity, we use the dense screen representation
+  // in the bridge and the TUI. The remote_ui module will translate this
+  // in to the public grid_line format.
+  void (*raw_line)(UI *ui, Integer grid, Integer row, Integer startcol,
+                   Integer endcol, Integer clearcol, Integer clearattr,
+                   const schar_T *chunk, const sattr_T *attrs);
   void (*event)(UI *ui, char *name, Array args, bool *args_consumed);
   void (*stop)(UI *ui);
   void (*inspect)(UI *ui, Dictionary *info);