Window resizing bug in piston backend

[mitchmindtree]

Seems to have been introduced in #850.

Looks like this:

[christolliday]

oh whoops, good catch, I actually saw this and assumed it was caused by some other change.
It looks like it's caused by not calling the correct next method for the window, I'm looking into a fix right now.

[mitchmindtree]

Closed by #856