688: Use only 1 and 4 samples for msaa-line example r=kvark a=cwfitzgerald
As 1 and 4 samples are the only universally supported values, switch the msaa-line example to only using them. In the future where users can determine the supported sample counts. See https://github.com/gfx-rs/wgpu/issues/804
Co-authored-by: Connor Fitzgerald <connorwadefitzgerald@gmail.com>