App: fixes to make it work here and clean things up a bit.

Most backends use same code for warmup, but leave at backend
just in case we want to do something special. No gloo required btw.
App: fix warmup issue (for the 100th time or so)
App: fix qt iffyness.

This took us sooo long to figure out. Slap on forehead.
Comments to other backends related to last commit

Also WIP in test simultaneuous
flake8
Merge pull request #4 from almarklein/multi-window

App: fixes to make it work here and clean things up a bit.