terminal: make deleteChars insert blank cells

When DCH (deleteChars) deletes cells from a line, cells are inserted
from the right. The new cells should not have the current pen style, and
should instead be an empty cell.

Add check for this in the existing test.

Fixes: alacritty/deccolm_reset

src/terminal/Terminal.zig

@@ -1282,7 +1282,7 @@ pub fn deleteChars(self: *Terminal, count: usize) !void {
     for (self.screen.cursor.x..self.cols) |x| {
         const copy_x = x + count;
         if (copy_x >= self.cols) {
-            line.getCellPtr(x).* = self.screen.cursor.pen;
+            line.getCellPtr(x).* = .{};
             continue;
         }
 
@@ -2810,11 +2810,17 @@ test "Terminal: deleteChars" {
     for ("ABCDE") |c| try t.print(c);
     t.setCursorPos(1, 2);
 
+    // the cells that shifted in should not have this attribute set
+    t.screen.cursor.pen = .{ .attrs = .{ .bold = true } };
+
     try t.deleteChars(2);
     {
         var str = try t.plainString(testing.allocator);
         defer testing.allocator.free(str);
         try testing.expectEqualStrings("ADE", str);
+
+        const cell = t.screen.getCell(.active, 0, 4);
+        try testing.expect(!cell.attrs.bold);
     }
 }