Upgrade WGPU to 0.15.0 and set nearest filtering for the framebuffer

[svercl]

I did two things here, first I upgraded WGPU to the newest version which only required adding in the alpha_mode to the SurfaceConfiguration, this should be okay, at least it is on my hardware.

Secondly, the framebuffer sampler now uses nearest filtering to keep the pixels crisp.

[carlfindahl]

I think this should be merged as it makes sense and would match the default renderer a lot more