This allows GNOME's redshift to work. Since wlroots gamma protocol
doesn't allow to read the gamma ramps fake empty ramps of the right
size. All users only use it to determin the gamma table's size.

Fixes #13

This is enough to see the monitors in the control panel with somewhat
correct mode information.

Future TODOs:

* use xdg-output protocol as well
* implement https://github.com/swaywm/wlr-protocols/issues/15
  so we can set rotation and scaling