glDrawArrays的功能：提供绘制功能，从数组数据中提取数据渲染基本图元。
定义
void glDrawArrays GLenum mode, GLint first, GLsizei count);
参数
mode
需要渲染的图元类型，包括 GL_POINTS, GL_LINE_STRIP, GL_LINE_LOOP, GL_LINES, GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN ，GL_TRIANGLES。
GL_POINTS：把每一个顶点作为一个点进行处理，顶点n即定义了点n，共绘制N个点
GL_LINES：连接每两个顶点作为一个独立的线段，顶点2n－1和2n之间共定义了n条线段，总共绘制N/2条线段
GL_LINE_STRIP：绘制从第一个顶点到最后一个顶点依次相连的一组线段，第n和n+1个顶点定义了线段n，总共绘制n－1条线段
GL_LINE_LOOP：绘制从第一个顶点到最后一个顶点依次相连的一组线段，然后最后一个顶点和第一个顶点相连，第n和n+1个顶点定义了线段n，总共绘制n条线段
GL_TRIANGLES：把每三个顶点作为一个独立的三角形，顶点3n－2、3n－1和3n定义了第n个三角形，总共绘制N/3个三角形
GL_TRIANGLE_STRIP：绘制一组相连的三角形，对于奇数n，顶点n、n+1和n+2定义了第n个三角形；对于偶数n，顶点n+1、n和n+2定义了第n个三角形，总共绘制N-2个三角形
GL_TRIANGLE_FAN：绘制一组相连的三角形，三角形是由第一个顶点及其后给定的顶点确定，顶点1、n+1和n+2定义了第n个三角形，总共绘制N-2个三角形
first
从数组缓存中的哪一位开始绘制，一般为0.
count
数组中顶点的数量.
功能
glDrawArrays specifies multiple geometric primitives with very few subroutine calls. It is possible to prespecify separate arrays of attributes and use them to construct a sequence of primitives with a single call to glDrawArrays.
When glDrawArrays is called, it uses count sequential elements from each enabled array to construct a sequence of geometric primitives, beginning with element first. mode specifies what kind of primitives are constructed and how the array elements construct those primitives.
To enable and disable a generic vertex attribute array, call glEnableVertexAttribArray and glDisableVertexAttribArray.
If an array corresponding to a generic attribute required by a vertex shader is not enabled, then the corresponding element is taken from the current generic attribute state. Errors
GL_INVALID_ENUM is generated if mode is not an accepted value.
GL_INVALID_VALUE is generated if count is negative.
GL_INVALID_OPERATION is generated if a non-zero buffer object name is bound to an enabled array and the buffer object’s data store is currently mapped.
GL_INVALID_FRAMEBUFFER_OPERATION is generated if the currently bound framebuffer is not framebuffer complete i.e. the return value from glCheckFramebufferStatus is not GL_FRAMEBUFFER_COMPLETE).
GL_INVALID_OPERATION is generated if recording the vertices of a primitive to the buffer objects being used for transform feedback purposes would result in either exceeding the limits of any buffer object’s size, or in exceeding the end position offset + size – 1, as set by glBindBufferRange.