用opengl绘制一个三棱锥,使用扇形绘制模式绘制了三棱锥的三个棱面，然后再绘制了三棱锥的底面，并启用了深度测试-Using opengl to draw a triangular pyramid, with three fan-shaped drawing mode to draw the three facet pyramid, and then draw the pyramid' s bottom three, and enable the depth test