最多支持到gl3，gl4以后无此函数。
glClientActiveTexture selects the vertex array client state parameters to be modified by glTexCoordPointer, and enabled or disabled with glEnableClientState or glDisableClientState, respectively, when called with a parameter of GL_TEXTURE_COORD_ARRAY.
glClientActiveTexture
texture
Specifies which texture unit to make active. The number of texture units is implementation dependent, but must be at least two. texture must be one of GL_TEXTUREi, where i ranges from 0 to the value of GL_MAX_TEXTURE_COORDS - 1, which is an implementation-dependent value. The initial value is GL_TEXTURE0.