screenSize view uniform should be an integer

[jespertheend]

Currently it's a vec4<f32> but since the screen size can't half pixels, an integer type would be more suitable here.

Although the z and w components of the vector are unused right now. We might want to provide something useful here.
Unity stores 1.0 + 1.0/w here for instance. If we decide to do this, an integer type wouldn't work.