Mouse input lag.

[lemmih]

Is the input lag inherent to the opengl backend or can it be improved?

Here's an example (running at 1/3 speed) where the circle position is set to the mouse position:

output.mov

[ogoffart]

With the Qt backend this cannot be reproduced, but I can reproduce it with the GL backend.

So it seems the problem is somewhere in the GL stack. Maybe @tronical have a better idea on where the problem lies and if that's fixable.
Could be anywhere in winit, glutin or femtovg or in our backend.

Reproducer

[tronical]

I've created #596 - this seems like a bug to me.