GazeContingentTutorial: Give this old chap some love...

Formatting cleanup, more instructive background color, better
explanation texts, fixed image loading.

Also use normalized 0-1 color space, and channel masks to
make the whole procedure a tad more fool proof.
DriftDemo5, DriftDemo6: Minor cleanups and improvements
KbQueueReserve(): Fix mixed usage of deviceIndex = [] vs. < 0

According to our docs, deviceIndex < 0 is the same as [] or
default, but KbQueueReserve() got confused when it was called
alternating with [], vs. -1. Fixed.