Add default branch to wgsl switch

[Bobo1239]

From the WGSL spec: "Each switch statement must have exactly one default clause."

Chrome/Tint emits the following error when compiling the shader:

Tint WGSL reader failure:
Parser: 35:5 error: switch statement must have a default clause
    switch (i32(input.vertex_index)) {
    ^^^^^^

naga doesn't yet validate the presence of a default clause but should do so soonish via gfx-rs/naga#1529.