gl.ReadBuffer(mode)
gl.ReadBuffer() specifies a color buffer as the source for subsequent gl.ReadPixels(), gl.CopyTexImage(),
gl.CopyTexSubImage(), and gl.CopyPixels() commands. mode accepts one of twelve or more predefined
values. #GL_AUX0 through #GL_AUX3 are always defined. In a fully configured system, #GL_FRONT, #GL_LEFT, and #GL_FRONT_LEFT all name the front left
buffer, #GL_FRONT_RIGHT and #GL_RIGHT name the front right buffer, and #GL_BACK_LEFT and #GL_BACK name the back left buffer.
Non-stereo double-buffered configurations have only a front left and a back left buffer. Single-buffered configurations have a front left and a
front right buffer if stereo, and only a front left buffer if nonstereo. It is an error to specify a non-existent buffer to gl.ReadBuffer().
mode is initially #GL_FRONT in single-buffered configurations and #GL_BACK in double-buffered configurations.
Please consult an OpenGL reference manual for more information.
#GL_INVALID_ENUM is generated if mode is not one of the twelve (or more) accepted values.
#GL_INVALID_OPERATION is generated if mode specifies a buffer that does not exist.
#GL_INVALID_OPERATION is generated if gl.ReadBuffer() is executed between the execution of gl.Begin() and the corresponding execution of gl.End().
#GL_READ_BUFFER