glClear -- 清除viewport的缓冲区
void glClear(GLbitfield mask);
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，可以使用以下标志位
GL_COLOR_BUFFER_BIT: 当前可写的颜色缓冲
GL_DEPTH_BUFFER_BIT: 深度缓冲
GL_ACCUM_BUFFER_BIT: 累积缓冲
GL_STENCIL_BUFFER_BIT: 模板缓冲
Remarks
The glClear function sets the bitplane area of the window to values previously selected by glClearColor, glClearIndex, glClearDepth, glClearStencil, and glClearAccum. You can clear multiple color buffers simultaneously by selecting more than one buffer at a time using glDrawBuffer.
The pixel-ownership test, the scissor test, dithering, and the buffer writemasks affect the operation of glClear. The scissor box bounds the cleared region. The alpha function, blend function, logical operation, stenciling, texture mapping, and z-buffering are ignored by glClear.
The glClear function takes a single argument (mask) that is the bitwise OR of several values indicating which buffer is to be cleared.
The value to which each buffer is cleared depends on the setting of the clear value for that buffer.