Don't allow negative width/height values

[radiovisual]

Seems the update to Electron v1+ has changed some behaviors in the Chromeless window, and now the rulers are able to resize themselves to negative widths and heights:

Before the update, the rulers would lock themselves out at a width/height of 0,0, and not the rulers get smaller than that.

[mikaa123]

I confirm: can reproduce ! I guess bumping Electron brings with it its share of surprises :)
Thanks for your help by the way 👍