
The logic behind the reference framebuffer:
 - by default there is no reference frame buffer set and Nux render to the opengl back buffer.
 - When a reference frame buffer is set Nux does its rendering into the reference frame buffer texture.
 - it is up to the caller to display the texture from the reference frame buffer into the back buffer.

To test this, a sample program would set a reference frame buffer and have Nux render a single color quad all over the window. Then the program would dump the texture in the reference frame buffer to memory and compare a few texels with the color that was used.

