ENH: send screenshot to EyeLink

[drammock]

Wishlist: During tracking, Eyelink lets you draw (bitmap) graphics in its interface, and will overlay a gaze direction cursor, so you can see what parts of your experiment screen the participant is looking at. Add EyeLinkController.send_screen or similar, as well as a way to put EyeLink into the correct mode for showing this (it is "non-plot-mode").

NB: EyeLink recommends the bitmap drawing be done in "offline" mode; whatever was last drawn in offline mode will be shown behind the gaze cursor during recording.

[drammock]

Note to self: not sure, but maybe this could be done through EyelinkController.image_buffer?

https://github.com/LABSN/expyfun/blob/master/expyfun/_eyelink_controller.py#L738