[glsl-out] Let snapshot tests request a specific GLSL desktop version.

Add a Option<u16> to the snapshot `Params` to let snapshots request specific
Desktop GLSL versions. The default is GLSL ES 3.10.

It would be more general to let the params select any GLSL version, but that
would entail making glsl::Version implement Deserialize and all that, which
seems like overkill.