SetAnaglyphStereoParameters: Implement 'CreateGreenOverlay' and 'CreateBlueOverlay'

Useful for encoding a secondary image in an otherwise unused colour
channel.
E.g., the green overlay window will store its image information in the
otherwise unused green colour channel of a red-blue anaglyph stereo
image, ditto the blue overlay window for the otherwise unused blue
channel of a red-green anaglyph.

The use case is to have a secondary monitor (e.g.,
experimenter/operator monitor) or recording device attached which
displays the overlay channel with additional information, e.g., eye
tracking data, whereas the subject/stimulation monitor only shows the
anaglyph colour channels.