term: "cache" pointer to current line

This adds a pointer to the first cell on the current line. This
pointer must be updated every time the row changes.

The advantage is mainly that PRINT doesn't have to call
grid_get_range(), which is fairly expensive.

terminal.c

@@ -169,6 +169,12 @@ term_cursor_to(struct terminal *term, int row, int col)
     term->cursor.linear = new_linear;
     term->cursor.col = col;
     term->cursor.row = row;
+
+    size_t len = term->cols;
+    term->grid->cur_line = grid_get_range(
+        term->grid, term->cursor.linear - col, &len);
+
+    assert(len == (size_t)term->cols);
 }
 
 void
@@ -253,6 +259,12 @@ term_scroll_partial(struct terminal *term, struct scroll_region region, int rows
         max(0, region.end - rows) * term->cols, 0, rows * term->cols);
 
     term_damage_scroll(term, DAMAGE_SCROLL, region, rows);
+
+    size_t len = term->cols;
+    term->grid->cur_line = grid_get_range(
+        term->grid, term->cursor.linear - term->cursor.col, &len);
+
+    assert(len == (size_t)term->cols);
 }
 
 void

terminal.h

@@ -66,6 +66,7 @@ struct grid {
     int offset;
 
     struct cell *cells;
+    struct cell *cur_line;
 
     tll(struct damage) damage;
     tll(struct damage) scroll_damage;

vt.c

@@ -678,11 +678,7 @@ action(struct terminal *term, enum action action, uint8_t c)
                 term_cursor_to(term, term->cursor.row + 1, 0);
         }
 
-        size_t cell_count = 1;
-        struct cell *cell = grid_get_range(
-            term->grid, term->cursor.linear, &cell_count);
-        assert(cell_count == 1);
-
+        struct cell *cell = &term->grid->cur_line[term->cursor.col];
         term_damage_update(term, term->cursor.linear, 1);
 
         if (term->vt.utf8.idx > 0) {