Allow taps on the left and right 20% of the screen to move back and
forth in the slides.  This seems like a decent compromise between false
triggering when people point at the screen and better responsiveness.
Don't display overheads in caching mode, they don't make sense once
images are cached.
Expose the HDMI reset and I2C interfaces to userspace.
Add the ability to configure HDMI output to de4ctl(8).

HDMI configuration code was obtained from miniboot's init.c and is a
blackbox.

Document de4ctl(8).