Support per-output -gradient

[himdel]

Until now, -gradient behaved as if -root was always present.

This changes gradient to paint per-output by default, add -root to use the original behavior.

(Note, on higher resoltions, this suffers from the same issues mentioned in #43, #42 and #28 .. but it also makes it possible to use gradient with multiple sub4k screens when only the total dimension is >4096.)