Externalize window grids in the basic architecture

The grids get allocated a handle (very naive at the moment) and these
handles are sent with every event to the external UIs.  Works for one
window at the moment.

Use neovim/python-gui#36 to test. It currently creates separate windows
for each grid. `:sp` seems to work, but `:vs` crashes.

src/nvim/api/ui.c

@@ -262,6 +262,10 @@ static void remote_ui_grid_resize(UI *ui, Integer grid,
   Array args = ARRAY_DICT_INIT;
   if (ui->ui_ext[kUIMultigrid]) {
     ADD(args, INTEGER_OBJ(grid));
+  } else if (grid != 1) {
+    // calling grid_resize with a grid other than the global when
+    // the remote ui is not managing grids should not send the event
+    return;
   }
   ADD(args, INTEGER_OBJ(width));
   ADD(args, INTEGER_OBJ(height));

src/nvim/screen.c

@@ -128,6 +128,11 @@ StlClickDefinition *tab_page_click_defs = NULL;
 
 long tab_page_click_defs_size = 0;
 
+/// The last handle that was assigned to a ScreenGrid. 1 is reserved for
+/// the default_grid.
+/// TODO(utkarshme): Numbers can be recycled after grid destruction.
+static int last_handle = 1;
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "screen.c.generated.h"
 #endif
@@ -5824,6 +5829,12 @@ void window_grid_alloc(win_T *wp, int doclear)
 
   grid_alloc(&wp->w_grid, wp->w_height, wp->w_width, doclear);
 
+  // only assign a grid handle if not already
+  if (wp->w_grid.handle == 0) {
+    wp->w_grid.handle = ++last_handle;
+    ui_call_grid_resize(wp->w_grid.handle, wp->w_grid.Columns, wp->w_grid.Rows);
+  }
+
   wp->w_grid.OffsetRow = wp->w_winrow;
   wp->w_grid.OffsetColumn = wp->w_wincol;
 }
@@ -5919,6 +5930,7 @@ void screenalloc(bool doclear)
 
   default_grid.OffsetRow = 0;
   default_grid.OffsetColumn = 0;
+  default_grid.handle = 1;
 
   must_redraw = CLEAR;          /* need to clear the screen later */
   if (doclear)
@@ -6230,7 +6242,7 @@ int grid_ins_lines(ScreenGrid *grid, int row, int line_count, int end,
     }
   }
 
-  ui_call_grid_scroll(1, row, end, col, col+width, -line_count, 0);
+  ui_call_grid_scroll(grid->handle, row, end, col, col+width, -line_count, 0);
 
   return OK;
 }
@@ -6282,7 +6294,7 @@ int grid_del_lines(ScreenGrid *grid, int row, int line_count, int end,
     }
   }
 
-  ui_call_grid_scroll(1, row, end, col, col+width, line_count, 0);
+  ui_call_grid_scroll(grid->handle, row, end, col, col+width, line_count, 0);
 
   return OK;
 }

src/nvim/types.h

@@ -23,7 +23,10 @@ typedef char_u schar_T[(MAX_MCO+1) * 4 + 1];
 typedef int16_t sattr_T;
 
 // TODO(bfredl): find me a good home
+typedef unsigned int GridHandle;
 typedef struct {
+  GridHandle handle;
+
   schar_T  *ScreenLines;
   sattr_T  *ScreenAttrs;
   unsigned *LineOffset;

src/nvim/ui.c

@@ -314,17 +314,16 @@ void ui_set_ext_option(UI *ui, UIExtension ext, bool active)
 
 void ui_line(ScreenGrid *grid, int row, int startcol, int endcol, int clearcol, int clearattr)
 {
-  size_t off = grid->LineOffset[row] + (size_t)startcol;
-
-  UI_CALL(raw_line, 1,
-          grid->OffsetRow    + row,
-          grid->OffsetColumn + startcol,
-          grid->OffsetColumn + endcol,
-          grid->OffsetColumn + clearcol,
-          clearattr,
-          grid->ScreenLines  + off,
-          grid->ScreenAttrs  + off);
-
+  size_t off  = grid->LineOffset[row] + (size_t)startcol;
+  int row_off = ui_is_external(kUIMultigrid) ? 0 : grid->OffsetRow;
+  int col_off = ui_is_external(kUIMultigrid) ? 0 : grid->OffsetColumn;
+
+  UI_CALL(raw_line, grid->handle,
+          row_off + row,
+          col_off + startcol,
+          col_off + endcol,
+          col_off + clearcol,
+          clearattr, grid->ScreenLines + off, grid->ScreenAttrs + off);
   if (p_wd) {  // 'writedelay': flush & delay each time.
     ui_flush();
     uint64_t wd = (uint64_t)labs(p_wd);