tui: Move screen state tracking to new "ugrid" module

The ugrid module implements a unicode "drawing" grid and is used to store TUI
screen state. Later this module will be reused in other layers.

src/nvim/ugrid.h

@@ -0,0 +1,40 @@
+#ifndef NVIM_UGRID_H
+#define NVIM_UGRID_H
+
+#include "nvim/ui.h"
+
+typedef struct ucell UCell;
+typedef struct ugrid UGrid;
+
+struct ucell {
+  char data[7];
+  HlAttrs attrs;
+};
+
+struct ugrid {
+  int top, bot, left, right;
+  int row, col;
+  int bg, fg;
+  int width, height;
+  HlAttrs attrs;
+  UCell **cells;
+};
+
+#define EMPTY_ATTRS ((HlAttrs){false, false, false, false, false, -1, -1})
+
+#define UGRID_FOREACH_CELL(grid, top, bot, left, right, code)           \
+  do {                                                                  \
+    for (int row = top; row <= bot; ++row) {                            \
+      UCell *row_cells = (grid)->cells[row];                            \
+      for (int col = left; col <= right; ++col) {                       \
+        UCell *cell = row_cells + col;                                  \
+        (void)(cell);                                                   \
+        code;                                                           \
+      }                                                                 \
+    }                                                                   \
+  } while (0)
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "ugrid.h.generated.h"
+#endif
+#endif  // NVIM_UGRID_H