Solve the delay problem when moving the viewer window

[PennyQ]

I found that when I try to move the Vispy viewer window in Glue there would be some delay phenomenon. Also it won't influence the whole data display but solving this problem could kind of polish the user experience.

[astrofrog]

I've created some minimal VisPy + Qt4 code (without glue) to show the issue here:

https://gist.github.com/astrofrog/aa1b4a490e61b82f9aa7

and have put an example here:

https://www.youtube.com/watch?v=wBg8SRpa580&feature=youtu.be

I'll contact the VisPy devs to see if anyone knows what this could be caused by.

[astrofrog]

I opened a VisPy issue here: vispy/vispy#1158

[astrofrog]

As for #40, this is fundamentally a Qt/OpenGL/MDI issue. Will be fixed only by allowing constrained layouts as described in glue-viz/glue-qt#40