select active texture unit
void glActiveTexture(GLenum 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_TEXTURE
i ranges from 0 (GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS - 1). The initial value is GL_TEXTURE0.
glActiveTexture selects which texture unit subsequent texture state calls will affect. The number of texture units an implementation supports is implementation dependent, but must be at least 48.
GL_INVALID_ENUM is generated if
texture is not one of GL_TEXTURE
i ranges from 0 to (GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS ...