Bitwise OR of masks that indicate the buffers to be cleared. The four masks are as follows. Mask Buffer to be Cleared 可以使用 | 运算符组合不同的缓冲标志位，表明需要清除的缓冲，例如glClear（GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT）表示要清除颜色缓冲以及深度缓冲 GL_COLOR_BUFFER_BIT The buffers currently enabled for color writing.