IE shows record button, but it doesn't work

Using Internet Explorer, I see the record button for CodeWorld programs. Clicking it gives:

SCRIPT438: Object doesn't support property or method 'captureStream'
codeworld.js (163,9)

It looks like canvas elements don't have captureStream on IE. The UI should detect that and not offer the record button if it cannot function correctly.

从 codeworld.js 第 163 行开始，这里 record 操作会在 Internet Explorer 中调用不受支持的 captureStream 方法。检查 UI 如何显示录制按钮，并添加 issue 中描述的功能检查。完成标准是：当 captureStream 不可用时，Internet Explorer 不提供录制功能，而受支持的浏览器仍保留该按钮。

由索引模型根据 Issue 内容生成。