Fix soft reset breaking default attribute

Fixes #2196

src/InputHandler.ts

@@ -1816,7 +1816,7 @@ export class InputHandler extends Disposable implements IInputHandler {
       this._terminal.applicationCursor = false;
       this._terminal.buffer.scrollTop = 0;
       this._terminal.buffer.scrollBottom = this._terminal.rows - 1;
-      this._terminal.curAttrData = DEFAULT_ATTR_DATA;
+      this._terminal.curAttrData = DEFAULT_ATTR_DATA.clone();
       this._terminal.buffer.x = this._terminal.buffer.y = 0; // ?
       this._terminal.charset = null;
       this._terminal.glevel = 0; // ??

src/core/buffer/BufferLine.ts

@@ -217,7 +217,7 @@ export class AttributeData implements IAttributeData {
   }
 }
 
-export const DEFAULT_ATTR_DATA = new AttributeData();
+export const DEFAULT_ATTR_DATA = Object.freeze(new AttributeData());
 
 /**
  * CellData - represents a single Cell in the terminal buffer.